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THE SCIENCE OF BRICKMAKING

THE SCIENCE
OF
BRICKMAKING:

WITH SOME ACCOUNT OF
THE STRUCTURE AND PHYSICAL
PROPERTIES OF BRICKS.

BY
GEORGE F. HARRIS, F.G.S.,

Membre de la Société Belge de Géologie, Paléontologie et
Hydrologie; Lecturer on Geology, The Practical Applications of
Geology, and Mineralogy, in the Birkbeck Institution, London;
etc., etc., etc.

LONDON:
H. GREVILLE MONTGOMERY,
43, ESSEX STREET, STRAND.
1897.

[ALL RIGHTS RESERVED.]

Printed at the Victoria Printing Works,
Stanstead Road Forest Hill S.E.

PREFACE.

The substance of this little work was first published as a series of articles in the British Clayworker, in 1895–96, and I am indebted to the courtesy of the Proprietor of that Journal for permission to reproduce them.

An attempt is here made to furnish some information of an elementary character on a special branch of technical education which has been seriously neglected in this country. But the reader will understand that it is only an elementary treatise. Its publication in serial form, where each article must, more or less, be complete in itself, has to a large extent determined the method of handling the subject, and I am fully cognisant of the drawbacks of the work in that respect.

At the same time, it is hoped that the book will be useful to the more advanced class of brickmakers and clayworkers generally, many of whom have expressed a desire to see the articles in this form.

Geo. F. Harris.

Birkbeck Institution,
Bream’s Buildings, Chancery Lane, E.C.
1st February, 1897.

CONTENTS.

THE SCIENCE
OF
BRICKMAKING.

CHAPTER I.
FLUVIATILE BRICK-EARTHS.

Let us go to Crayford, near Erith, or to Ilford, in Essex, and take a superficial glance at some of the brickyards found at those places; in particular, let us ascertain a little concerning the earths employed. We find in one brickyard a series of stiff brown or bluish clays, interstratified between sandy clays or “loams,” with thin brownish partings. In another, the loam will become very sandy, and the earth light, with a slight greenish tinge. A third has thin pebble or gravel beds developed, or small stones sparingly scattered in the clays and loams on certain horizons. A fourth contains, in addition to some of the beds above described, a lime-clay or marl[1] with small pellets of chalk. It will be noticed on entering the yards that these various horizons, or beds, as they are conveniently termed, are disposed in regular lines or layers, more or less horizontal; in other words, the beds are “stratified.” On the face of the working being dug into, it will often be found that these thin beds, a few inches or feet each in thickness, vary in depth, and frequently disappear altogether, or “thin out,” whilst, on the other hand, a bed only a few inches thick may become as many feet, and new beds are found to be developed. A pure sand may in like manner become loamy on being dug into, and, on being further developed, pass insensibly into a stiff clay. And many other changes take place into which we will not enquire at the moment. Suffice it to say, that in such brickyards the strata are very locally developed, though it follows from the circumstance of their existence for so many years, that what changes have taken place, to some extent compensate each other, so that the material is still an earth suitable for making bricks. Again, certain beds of much economic value may be more persistent than others, both in character and development. Having noticed all these things, we perceive a couple of men digging with care into the brick-earth, and presently they bring some objects to us which we have no difficulty in recognizing as the remains of the lower jaw of an elephant’s skull. Returning to the spot where they were exhumed, the upper jaw and tusks also are uncovered. To the clay workers these things are well known; in their time they have found many similar skulls of animals in the brick-earth; but they know next to nothing concerning them, or how they got there. Another expedition to the same localities may yield the remains of rhinoceros, the musk sheep, grizzly bear, hippopotamus, reindeer, and many other animals. A fine series of the remains of these, obtained from the brick-earths of the valley of the Thames at several points, is exhibited in the geological department of the British Museum (Natural History), South Kensington, and more or less complete skeletons obtained from the same source may be found in other, and local museums. One of the most interesting points concerning these remains is that so many of the animals represented in the brick-earths are of extinct species—there are no species included in this latter category of precisely similar kinds to animals now living, Thus the elephant was different to modern elephants; we know, from remains found elsewhere, that it was clothed with wool. The same also with the rhinoceros. The reindeer no longer lives in this country, being confined to northerly latitudes; whilst the musk sheep is a denizen of the Arctic regions, and the hippopotamus is restricted to the tropical or sub-tropical climes. But we might continue for a long time expatiating on the character of the very numerous mammalian remains found in our common brick-earths. What a curious assemblage of animals! It is wonderful to contemplate the time when the reindeer and musk sheep lived side by side with the elephant and rhinoceros on the site whereon London now stands.

That is not all, however. In the same brick-earths and gravels, tools (flint implements), fashioned by the hands of man, are also frequently discovered, and in one place at Crayford, the spot whereon flint implements were manufactured has been ’lighted upon. Each flake chipped off has been collected and pieced together, and the shape of the original flint has thus been determined. Clearly, from this evidence, the earth from which millions of bricks have been made has formed since primæval man (and with him the animals alluded to) inhabited the valleys of the Thames and its tributaries. It is interesting, too, to reflect on the circumstance that the materials upon which many of these facts of great philosophical significance are based, have been collected through the instrumentality of the workmen. Palæontologists are proud to acknowledge that; their debt of gratitude to the intelligent and persevering men can never be fully repaid.

Pursuing the matter still further, we discover a quantity of shells, blanched and very frail—they seem to be deprived of much of their original substance, so to speak; their entombment in the brick-earth has taken all the natural colour out of them. Studying these, we soon ascertain that they belong to land snails and mollusca which inhabit fresh water. Living representatives of the same species are, with few exceptions, found in Kent and Essex.

Putting all this evidence together, we come to the conclusion that the brick-earths alluded to accumulated in the channel of a river; they are found above the present level of the Thames, for the simple reason that they have been elevated into that position partly by earth movements and partly by the channel of the river being cut deeper by natural causes, of which abundant proof will be adduced. The snails were washed down from the land by freshets, or caught by the river in flood; the elephant, rhinoceros, hippopotamus, and musk sheep were overcome, perhaps, by floods, drowned, and subsequently covered up by the mud of the swollen current. We can imagine that the savage hunter, in his canoe, attacking the animals swimming in the river, loses his tomahawk, or his frail bark may be upset, and he is striving to gain the shore for dear life. Or, it may be winter time; the river is frozen over, and he is cutting a hole in the ice with his flint chisel wherein to fish; his hands are benumbed, and he loses his grasp of the tool; it falls into the water, to be discovered in the brick-earth by one of our intelligent friends. Truly, the revelations of the brickyard enable us to construct a picture of one of the most interesting phases of the past history of the Earth.

We have given an outline of the evidence upon which certain brick-earths in the Thames valley are proved to be of fresh-water origin—to have accumulated in quiet reaches of the river, and at other convenient spots along its course—but we have used that as an illustration only; phenomena of precisely the same character are manifested in nearly all river valleys in this country, especially those in which the bottom of the valley has only a slight gradient down to the sea.

The brickmaker may ask: What is the practical bearing of these observations? What difference does it make to us whether the earths we use are of fresh-water, lacustrine, or marine origin? All the difference in the world, from the points of view of structure, composition and suitability of the earths, and especially of their distribution over the face of the country. How much easier it is to value an extensive brickmaking property when you feel perfectly certain as to whether the face of earth as shown in the pit will die out on being worked into for a few yards, or whether it will be persistent throughout the whole of the property to be valued. Better still, when your knowledge enables you to state definitely whether the quality of earth now being worked in a pit is likely to continue the same, or whether it will get better, or worse. The disposition of the earths, in some instances, is so clear that no brickmaker with an eye to business could fail to trace their extent over his property. But this is not often the case, for the earths being used are for the most part covered by a superficial mantle, or overburden, which masks the true character of the beds beneath. A very slight acquaintance with the principles of geology overcomes these difficulties as a rule; and we are about to lay down the elements of these principles, so far as they apply to the immediate subject in hand. By seeing why it is the beds of brick-earth vary in structure and composition we shall be in a better position to make forecasts of their general behaviour.

In regard to fluviatile deposits, it goes without saying that every river flows along a general depression more or less pronounced, called a valley, and that this valley is bounded physiographically by a ridge, except in the region of its entrance to the sea or lake, or, if a tributary, of its joining a main stream. The watershed of a river and its tributaries includes and comprises what is technically termed the “river basin.” All valleys are, in the end, the result of denudation taking place in them. In other words, on the birth of a valley a very slight depression or other physical feature determined its general direction for the time being, but the little rivulet once being formed proceeded, through the medium of the “agents of denudation,” to carve out its channel more clearly, and eventually to eat into the rocks over which it flowed, until a large valley had been formed. The “agents of denudation” in river valleys may be summarised as rain, snow, ice, heat, and wind, and their general effect on rocks is called “weathering.” We need not stop to enquire into the precise methods adopted by these agents in accomplishing their work; it suffices at present to say that the rock destroyed or broken up is removed by the running water constituting the rivulet, stream, or river, as the case may be. Some of the material is chemically dissolved in the water, whilst another and larger proportion is taken away in suspension, or is said to be dealt with mechanically by the river. The agents of denudation do their work very slowly, as a rule, and yet no one who stands on London Bridge and contemplates the swollen stream laden with muddy sediment passing under it after a few days’ rain, could say that they are not doing their duty effectually. To give some idea of the quantity of sand, gravel, and mud removed from the land through the medium of rivers, we may remark that the Mississippi discharges into the Gulf of Mexico annually a mass of earthy matter equal to a prism 268 feet in height with a base area of one square mile. In regard to denudation by chemical means we may say that the Thames carries past Kingston 19 grains of mineral salts in every gallon of water, or a total of 1,502 tons every 24 hours, or 548,230 tons every year; this is not taking into account the muddy sediment, gravel, &c., annually sent down to the Nore, which must be infinitely greater in quantity.

Enough has now been said to show that stupendous quantities of mineral matter derived from the destruction of the land are sent down to the sea by natural agencies, and we may at once state that a very large proportion of this, which finds a resting-place in and about the mouths of the rivers and their backwaters, is material suitable for brickmaking at places where it is obtainable. Enormous quantities of muddy sediment, sand and gravel, however, never reach as far as the sea with great rivers. This material is arrested at sundry convenient spots, and, as a rule, forms excellent brick-earth.

Fig. 1.—Formation of Brick-earth in a river valley.

See [Fig. 1], which represents part of a river of slow current with three bends, A, B, C. The water is flowing in the direction indicated by the arrows; and it is part of the mechanics of such a river that in rounding a bend its velocity is greatest (and its eroding power also) at the outer portions of the curves approximately indicated by the arrow points. The water “wheels round” such portions of the curves, and “marks time” at the points x x x, and, indeed, its progress may be altogether arrested for a time at the latter places. Now the transporting power of a river is its velocity, and, naturally, the greater the velocity, the coarser will be the fragments or particles of rock carried along. It is interesting in this connection to quote the figures calculated by Mr. David Stephenson, giving the power of transport of different velocities of river currents:—

These figures[2] have greater interest for us than in the connection at present used, as will be noticed hereafter. We have seen that in rounding the bends ([Fig. 1]) A, B, C, different portions of the stream possess different velocities. We know it is charged with sediment and stones all the time. The tendency, therefore, will be for the large stones and coarse detritus to go round the outer side of the bend, to bombard the banks near the points shown by the arrows, and to erode the channel deepest in those situations; whilst a goodly proportion of the fine muddy sediment will find its way to the quiet and shallow parts near x x x, and in course of time become deposited there, whilst the main course of the stream is eating its way and shifting its course as indicated by the dotted lines a a. This action proceeds, it may be, until the course of the river becomes straighter, as shown by the dotted lines b b, when the whole of the loop B D is abandoned, its former course there being evidenced by pools of water and irregular heaps of gravel, sand and mud. The reader will now see that the whole of the land marked x x x has been formed of sediment brought down by the river, and in the majority of cases such fine silt and sandy mud or clay is specially suitable for brickmaking—many of our largest brickmakers obtain their material from such a source. It should be observed that the valley, as shown between the lines v v, may be two or three miles in width, and it is often much more, so that the actual amount of land made by the river at x x x may be several thousands of acres in extent.

Now as to the practical application of the foregoing observations. In the first place, it will be seen that such deposits of brick-earth as are made in this manner cannot be very thick, their total thickness perhaps, resting on the bottom of the valley, not being more than 20 feet, and it is frequently much less. The next thing to be noticed is that they must be very variable in character, a bed changing perhaps every 100 feet or so horizontally, and more often every few feet. Individual beds must of necessity be very irregularly developed under the circumstances. The velocity of the stream being greater at certain seasons of the year than at others, we frequently find some such section as the following developed:—

Fig. 2.—Section of Fluviatile Brick-earth.

a = Mould and soil, of no use to the brickmaker.

b = Sandy clay, with a large proportion of sand; useful for moulding or incorporating with the “fat” clays below for brickmaking.

c = Gravel bed, lenticularly developed; suitable for mending roads, paths, &c.

d = Sandy clay; similar to b.

e = Thin bed of marl, with a fair proportion of lime.

f = Sands and small pebbles, irregularly stratified (false-bedded).

g = Laminated sandy clay.

h = Stiff clay; can be mixed with f and passed through the pug mill.

i = Sand; an irregular bed of very local occurrence.

j = Gravel bed, with much sand.

The above is typical of deposits accumulated in river valleys; it is different in character to deposits laid down in the sea (as will presently be described); the section exhibits very different classes of brick-earth also, and yields a totally different kind of brick to that obtainable from brick-earths of marine origin. The importance of the question of origin of a brick-earth, therefore, is just beginning to dawn upon us. Many rivers are noted as having throughout a long period of time wandered from one side of the valley to the other (by the process depicted in [Fig. 1]) several times, in which cases the brick-earth sections relating to them are liable to still greater variation. The reader would perhaps be very much astonished to find how much is known concerning peregrinations of that description in regard to particular localities, by competent authorities—usually field geologists.

We come to another important point in regard to river deposits. The ceaseless flow of the river, and the abrading action of the large stones rolled along at the bottom of its channel, tend to cut the latter deeper and deeper, and we have excellent evidence that most English rivers once flowed at a greater elevation in their valleys than they now do. In consequence of this, the brickmaker may find his pit somewhat higher than the neighbouring river, which at an earlier stage of its existence made his brick-earths. To a certain extent, small earth movements, as previously explained, are also undoubtedly responsible for many of these brick-earths now being at a considerable elevation above the surface of the river. This phenomenon is illustrated in [Fig. 3].

Fig. 3.—Section across a river valley, showing formation of terraces of gravel and brick-earth.

This type of disposition of fluviatile deposits is of common occurrence. We will assume that the valley is carved out of clay (shown by horizontal lines and dots). On both sides of it, and at the same relative heights, are two masses (marked 1 and 2) of brick-earths and gravels running along so as to form two distinct broad terraces. These beds were laid down when the river, in flood, though occupying only a small portion of the valley, was approximately of the height shown by the dotted lines a b. Denudation has been hard at work, however, since then, and only vestiges of these beds clinging to the sides of the valley, as shown, remain. At a later period, and coming on towards modern times, the broad expanse of beds (comparable in disposition with those depicted in [Fig. 2]) some miles in width, marked 3, were laid down, and we notice the river channel, as it now is, cutting its way through them. Thus it comes to pass that brickyards may be situated in terraces one above the other; and what is much more important, the brick-earths may vary very widely in quality along these horizons, those in 1 differing from 2 and both from 3. The brickyards may be quite close to each other, and to the unscientific eye the earths are of similar appearance, but they do not yield the same class of brick, and no one seems to trouble to enquire the reason why. These differences have resulted primarily from the materials having been derived from other collecting grounds, other watersheds, than those comprised within the basin of the river as at present constituted. They are the inevitable accompaniment of the evolution of the river system, and throw light on successive phases of the past history of the stream and its tributaries. For us, as we have seen, they possess considerable practical value of the first importance in selecting the site for a brickyard.

Apart from differences of the character just described, serious alterations sometimes take place on these brick-earths being traced higher up the valley, and indeed an excellent brickmaking material may become absolutely worthless in that respect, for the reasons about to be explained. The reader will agree that neither stones nor sediment can travel up a valley, and he will understand that no sediment can be found in the valley earths other than that derived from the destruction of rocks within the watershed of the river system, to which the valleys belong, or did belong, at the time the earths were formed. We desire to put the case in a very simple light, so as to be clearly comprehended. Let us contemplate [Fig. 4].

Fig. 4.—Map shewing river basin, with geological formations depicted.

Here we have represented a river basin, the limits (watershed) of which are indicated by a sinuous dotted line. Three geological formations are found therein; in the upper reaches of the main river is a series of clays marked A; a large tract in the middle, B, is sandstone; and the lower part, C, is occupied by limestone. Seeing that nothing but clay crops out in the part A, it follows that the deposits of the river in that region must be principally of an argillaceous character, to the point a. On flowing over the sandstone B, the main stream, already charged with clay particles, will be mixed with sand; the proportion of sand increases as the first large tributary (b) to the east is encountered, and is considerably augmented as the still more important tributary (b) to the west enters it. The superficial deposits in the valleys of the area B will likewise be very sandy and perhaps gravelly at b b, but at c c¹ the sands and gravels will be mixed with much clay. On passing over into the area C, much carbonate of lime is added, though the larger proportion denuded from the rocks is taken away, chemically, in solution. Nevertheless, nodules of “race” (lime concretions), limestone pebbles, and perhaps chert and flint gravel will come upon the scene at about the point marked e. At d the deposits would principally consist of gravel and impure marls. To sum up, the clays at a would no doubt be too stiff of themselves to make good bricks; similarly the beds at b b would be nothing but sand, though these might be made, with a little judicious treatment, into a species of fire-brick; at c we should find alternating loams and clays suitable for turning out fair bricks; at c¹ the beds would be more variable in character and more locally developed; they would consist of thin beds of sand, clays, loams and gravels (principally sandstone fragments), which as a whole might be made serviceable, though difficult to deal with; nothing of much use to us would come from point d, nor bordering the tributary running over C; there would be too much lime present, though a trade might be started in basic bricks should there be any demand for them in the neighbourhood; this, however, would only pay under extremely favourable conditions. At e there may be a mixture of all the foregoing deposits, and providing the beds above were easily weathered and thick beds of loam were thus fairly well developed, good sites for brick-earth might be found. The point e might possess this advantage over the other sites mentioned, viz., that marls would no doubt be present, and thus no necessity should arise for grinding lime to be incorporated with the brick-earth; the only danger would be that lumps of limestone might be too numerous—especially if c were a hard limestone.

The general character of the deposits might be slightly modified by mineral matter brought up in springs and thrown down at convenient spots.

CHAPTER II.
LACUSTRINE AND FLUVIATILE BRICK-EARTHS.

The great variability of brick-earths deposited in river valleys is reflected to some extent in those laid down in lakes, though the size of the latter is frequently a controlling factor. The chief difference consists in the broader expanse of the sediment laid down—especially in large lakes—and variation in structure is not so noticeable horizontally. Let us consider a simple case in which a lake is fed by a large river bringing down abundant sediment. The lake acts as a species of settling tank, and the method of deposition of the sediment by the river is mainly guided by the velocity of the stream. The tendency under normal conditions is for the river to commence parting with its sediment immediately on entering the lake. The detritus alluded to is only held in suspension by the velocity of the water; when the latter is checked, as on entering the lake, the grosser pieces subside, and as its rapidity becomes progressively curbed, medium-sized fragments are compelled to give way, until at last only very minute particles are left in the water. In due time most of these also are deposited. Thus gravel is laid down before grit, grit before sand, and sand before clay.

If the velocity of the river always remained the same, we should be presented with thick accumulations of the same character in sharply defined areas. But it is always changing. With every storm and every steady rain the motion of the river becomes greatly accelerated, with the result that the deposits for the time being are deposited farther out in the lake than in more quiescent periods. In this way we may have a gravel thrown down on sand, sand on clay, and so on.

From the foregoing observations it will be gleaned that, in general, deposits in large lakes are more persistent in character than are river deposits; indeed, in very large sheets of water, as Lake Superior, Lake Erie, &c., they are in this sense more comparable with sediment of marine origin.

The practical value of this knowledge hinges on the correct determination of the origin of the deposits, and it is not always easy to identify a brick-earth of lacustrine origin. In all probability the tyro, on meeting one, would be disposed to regard it as a river deposit pure and simple. The valuation of a brick-earth property under such circumstances would thus be greatly in favour of the prospective purchaser; but it would be disastrous for the seller. A random section, except in the case of a very large lake, would show gravels, sands and clays in much the same manner as the river deposits described in the last article of this series. But, as previously remarked, on the whole they would be more continuous and persistent, and what is quite as important, the mineral composition of each stratum would be equally homogeneous when traced over wide areas. The geologist distinguishes a lacustrine deposit from one of fluviatile origin more from its mineral constitution and the general disposition of the beds, as ascertained by mapping, than from evidence afforded by fossils—these latter for the most part being similar to those found in the deposits left by rivers.

The well-known brick-earth called “Reading mottled clay,” so extensively developed on the outskirts of the London basin, and in the Isle of Wight and Hampshire generally, furnishes a good example of a lacustrine deposit. Many millions of bricks are made from this bed every year, and in some parts of the districts mentioned the stratum is thick and extensively developed. It is pure enough to be suitable for terra-cotta manufacture here and there. No one who had seen this remarkable deposit could possibly fail to recognise it again. The natural colour of the clay when damp is brilliant red, scarlet or crimson, in large blotches and patches mottled tea-green and yellow, and locally white.

We have been intensely amused to note the efforts in recent years to obtain possession of a few acres of this coveted deposit for brickmaking in divers localities. Not long since we visited a large brickmaking establishment where these Reading plastic clays are actively raised and used, the works being situated four miles from the nearest railway. There were no other brickworks between it and the railway line, and there was no water accommodation. Enquiry revealed the fact that the greater part of the intervening land belonged to the same landowner as the ground where the brickyard stands, and that no difficulty was apprehended of the owner letting out such intervening land for the same uses and on the same terms if other brickyards were contemplated. The proprietor of the brickyard in question volunteered the information that the reason he started so far from the railway was because the earth at the point selected was the only kind suitable for brickmaking in the neighbourhood. We then questioned him as to his knowledge of the brick-earths in the district, and eventually elicited the fact that he chanced upon the spot selected, without any reasoning therefor, and commenced operations. As a matter of fact, precisely the same clay extended from his works all the way to the railway line, and had he known anything whatever of the geology of the district (even the merest boy’s knowledge of the subject), he would have seen how to save that four miles of road carriage. What prevented him from knowing the fact was a thin mantle of gravel and soil about four feet in thickness, which covered the plastic clay in the area generally, except in the immediate vicinity of his brickyard. That was in reference to a lacustrine deposit—the Reading plastic clay—and shows the value of knowing something of its persistent character; if it had been a river deposit there would not have been so much room for wonderment.

To give some idea of the extent of that particular horizon, we may say that not only is the plastic clay alluded to found so extensively in the London and Hampshire basins, it is even more expanded in the north-eastern parts of France, and is there as much utilised as on this side of the Channel for brickmaking.

Lacustrine deposits are sometimes of enormous value to the clayworker, on account of the general purity of the clays. This is more particularly the case when the material deposited is in part or wholly derived from chemical disintegration of granitic rocks, as in the celebrated Bovey Heathfield clays near Newton Abbot, so well described in a small pamphlet by Mr. S. Smith Harvey. Here an experimental boring proved the clays to a depth of 130 feet with no signs of exhaustion. In the divers clay-pits but a small proportion of waste is found, the different levels vary in composition, and, like almost all thick clays, improve in quality as the depth increases. The strata are very irregular towards the surface, due perhaps to the action of local freshets in the final periods of the history of the lake. These clays are extensively employed for the manufacture of stoneware pipes, facing and other bricks, fire-bricks, etc. They constitute a somewhat remarkable exception to the class of clays laid down in lakes, as a rule, and, as will have been observed, are of enormous thickness.

We have very little to say in regard to estuarine brick-earths; as might readily be anticipated, they are intermediate in character between fluviatile and marine deposits, and approach the one or the other according to position in the estuary. On the whole, they are variable in character, individual beds being thin. The strata frequently contain abundant plant remains (pieces of wood, etc.), and, except in the case of large rivers, are not noted for yielding very good brick-earths. Sometimes, however, the quality of the clays is not bad, as instance the bricks made in Lincolnshire and Northamptonshire from Jurassic Estuarine clays.

CHAPTER III.
MARINE BRICK-EARTHS.

Turning to brick-earths of marine origin, we may say that these constitute by far the largest class of deposits from which bricks are made in this country, and it will be useful to deal with their origin in some detail. If we attentively watch the action of the weather on a friable sea-cliff we notice that large pieces tumble at intervals on to the beach, and in due time these are washed away by the waves, thus encouraging more to fall when the time is ripe. This process of denudation each year takes tens of thousands of tons of sandy clays and the like from the beaches around our islands. Large pieces of rock, too, are detached by the weather, and eventually succumb to wave action. During storms large stones are hurled against the cliffs, and the general effect of this bombardment is to wear them away, and reduce them to powder and sand grains with all possible expedition. No one who has not seen the waves at work at such times can have any idea of their tremendous power of moving blocks of stone many tons in weight. During calm weather the slight movement of the waves on the beach is manufacturing tons and tons of sand. A mass of gravel falls from the cliff; the finer particles are floated away at the earliest opportunity; the angular stones have their rough projections knocked off by striking against each other; and the incessant movement up and down the beach slope reduces the rough stone to a pebble, all the time the particles thus shaved off are taken out to sea for greater or less distances. If the cliffs are of limestone, or similar rock, both chemical and mechanical methods of denudation come into play, and considerable quantities of lime, &c., are taken away by the sea water in suspension and solution. Large quantities of lime are daily added to the sea through the agency of rivers also.

Now, what becomes of these vast quantities of detritus furnished to the sea? That depends on the shore currents at the particular locality. If there is not much of a current, the larger grains of grit and sand are soon separated from the rest, and fall to the bottom, whilst the clays are taken farther out to sea before being laid down. But, in any case, the reader will readily perceive that marine deposits must of necessity be on a grander scale, and of a much more substantial character, as a rule, than river, lacustrine, or estuarine deposits. By their mode of origin, too, they must be more homogeneous, whilst they are frequently several hundreds of feet in thickness. In their process of deposition they were not influenced by every storm and freshet; nothing short of great earth-movements in process of time, or some other equally grand phenomena, could disturb the even tenour of their existence. How different to the comparatively insignificant strata formed by the other methods alluded to!

Take samples of brick-earth of fluviatile origin at intervals and analyse them; no two analyses will be alike, except by a most remarkable coincidence—more by accident than otherwise. On the other hand, take a thick marine clay, and compare its chemical composition as ascertained at the present time with that of it made, say, 20 years ago in the same brickyard, and the analyses will, in most instances, be practically identical—at any rate, so far as they may be of use to the brickmaker.

A brickmaker using a marine clay possesses innumerable advantages over another employing brick-earths due to river action. It is no uncommon thing for a marine clay—say, 300 feet in thickness—to continue across country for hundreds of miles, stretching from the North of England to the South, and over into the Continent, save for the slight break occasioned by the scooping out of the English Channel. The composition of the Oxford Clay, from which the well-known bricks at Peterborough are made, does not differ in the slightest degree, so far as suitability for brickmaking is concerned, from the Oxford Clay of Bourges or Chateauroux, in the centre of France, or indeed at almost any other point en route. With marine beds it is possible to deal with the matter on broad lines, but it is not so with any other class of deposits.

If a marine clay in a specified locality is found to be unsuitable for bricks at one point, by reason of the presence of too much lime, it would be a phenomenon if clay along the same geological horizon did not present the same unfavourable features at every other point within the district. The homogeneous composition, both from mineralogical and chemical standpoints, of thick marine clays renders them of special use to the brickmaker. Having by sundry processes, after infinite labour, produced a certain class of brick from such an earth, he does not as a rule have to materially modify those processes as the earth is dug into to continue manufacturing the same brick. He is dealing with an earth which, comparatively speaking, is a constant quantity—when the clays are thick, and no lines of bedding are distinctly visible.

We find that a rooted conviction exists in many brickyards that clays of marine origin are no good for brickmaking, because (so the opinion runs) they always contain so much salt. It is wonderful that such ignorance prevails, when the slightest acquaintance with the subject would teach otherwise. It is perfectly true that such deposits might have contained salt during and for some time after deposition, but it is absurd to suppose that their marine origin has anything to do with the presence of common salt in the clay at the present time. Salt is soluble in water, and has been removed from such clays by the percolation of underground water in 99 cases out of a hundred. Indeed, as a matter of experience, we find that salt is most commonly found in beds of lacustrine origin, or those laid down in enclosed portions of the sea, for reasons we need not enter into at the present moment. Of course, when material is taken from the sea-shore to make into bricks, a considerable quantity of salt is manifest, but that is a totally different thing to the clays deposited—we should not like to say how many thousands of years ago. Clays of all kinds, however, may be impregnated with salt (as in parts of Cheshire), owing to the proximity of other beds containing that mineral; also by the percolation of underground water with much salt in solution.

To give some idea of the antiquity of the Oxford Clay alluded to—and that is quite a “young clay” geologically speaking—we may remark that at the time it was laid down not a single species of animal existed like those now living. The only mammals found, very small and very lowly organised, were like kangaroo rats; the birds were more like flying reptiles than anything else; it was the age of reptiles, and enormous, unwieldy brutes swam in the water or floundered about on land; huge sharks abounded, and armour-clad fish of kinds very different to those now existing roamed the sea; even the “shell-fish” were not altogether like modern ones; whilst the plants find their nearest modern analogues in the wilds of Australasia. No elephants, tigers, lions, bears, or dogs lived then, and the face of Nature wore a totally different aspect to what obtains at the present time in any part of the globe.

And this seems a fitting opportunity to the writer to put on record the fact that many of the most wonderful remains found in the Oxford Clay and the neighbouring Kimeridge Clay are due to the discoveries of brickmakers. Without their valuable aid scientists would be quite unable to clearly depict the life of those remote epochs. We have mentioned Peterborough; some most interesting remains have been found in the clays near that town during the past few years. To appreciate this let the reader visit the fossil reptile gallery of the British Museum (Natural History), at South Kensington. One of the most recent acquisitions, set up a year or two ago, is the skeleton of a young Plesiosaurus—without doubt the most perfect specimen in the world of its kind—from Peterborough. The Plesiosaurus was a large swimming reptile, with paddles, and a long neck.

We mention these things not only to instil philosophical interest in such brick-earths, which may be reflected upon after business hours, but to impart some idea of the extreme remoteness of the epoch from the human point of view, and to insist on the immensity of the intervening time throughout which circulating underground waters—even in such an impervious material as stiff clay—may have exerted chemical action. The “mineralisation” of the fossils is an eloquent witness of the effect of such changes. The reader will perceive from this that there is scant possibility of soluble salts being present in such marine clays; and the geological circumstances are fully borne out by the results of hundreds of chemical analyses of thick marine clays.

The invertebrate fossils more particularly testify to the marine origin of the clays, and are thus invested with considerable practical interest. The man whose duty it is to determine the persistence, or otherwise, of valuable marine brick-earths has thus a much easier task than when called upon to decide the value of a large tract of land for brickmaking purposes, of fluviatile origin. Finally, brick-earths do not, except in extremely rare instances, vary materially in character when dug into horizontally, thus every opportunity is afforded to the manufacturer for making an unvariable quality brick, tile, or drain pipe. It should be borne in mind, however, that these clays often weather a brown colour, which on being dug into changes to a bluish-black tint, the latter being the unaltered and best portion as a rule. The only practical advantage the worker of a superficial river deposit possesses over his neighbour using thick marine clay is in the great range of variation in materials disclosed in the former kind of pit. By judiciously mixing the different beds he may be able to live well where the worker of marine clays, especially where the clay is too stiff, or contains too much lime, “comes to grief.” A good marine clay is a great boon, a bad one cannot be remedied other than by the sacrifice of much money.

CHAPTER IV.
THE MINERAL CONSTITUTION OF BRICK-EARTHS.

There cannot be any question that the applicability or otherwise, of an earth for making good bricks, to a large extent depends on the mineral constitution of that earth. A chemical analysis of a sample of such earth will tell us how much silica, alumina, lime, iron, etc., is present therein, and this information is frequently of great value when given by a scientific chemist; but it does not tell us the state in which those constituents exist in the earth—an essential desideratum, if we are to understand the scientific aspects of the question of burning in the kiln. Further, the size of the granules and particles composing the earth is well worth knowing, as we shall presently see. It is a great mistake to imagine that all clays are essentially chemical deposits. The majority of them have been in part derived from chemical disintegration, it is true; but the resulting deposits contain so much also that is purely of mechanical origin, that the behaviour of the whole is materially modified, from a metallurgical point of view. Take one ingredient, for example—say, silica. That may exist in a brick-earth in a variety of ways, both in a free and combined state; but its behaviour in the kiln is largely dependent on the particular form assumed, not only whether it is free or combined, but as to how it is combined. In a certain sense, it is very doubtful whether even in the best-burnt brick much of the raw material becomes chemically combined; a sort of agglutination takes place locally, as is clearly shown by the microscope; at such points true fusion undoubtedly takes place, and there may be actual chemical combination. In the vast majority of cases, however, such fusion or possible combination is of an extremely partial and elementary character, whilst it hardly exists in the average “rubber.” The microscope shows that even in the hardest burnt brick there still remain enormous quantities of what may be termed mineral grains, that have by no means succumbed to the burning process. The edges of the grains may occasionally be seen merging into the more or less vitreous ground mass in which they are embedded, but beyond that they appear tolerably fresh, and their action on polarised light remains unimpaired.

We did not intend to say anything yet concerning the microscopic structure of bricks—that will be gone into in a subsequent chapter; but we thought it useful to state the foregoing elementary facts in order to endeavour to uproot a conviction that seems to be very firmly grounded—viz., that the chemical composition of a brick-earth imparts an accurate idea of the possible active agents, on the earth being subjected to the kiln. As a matter of fact, some of these would-be agents are imprisoned in the mineral grains and particles that have not become involved in the partial melting or agglutination of the mass, and might as well not be present in the earth for any work they may accomplish either for good or for evil. There is greater probability of the bulk of these grains and particles being of active service when they are ground up exceedingly fine; but the clayworker’s idea of “fineness,” as demonstrated by what passes through an ordinary clayworking mill, and “fineness” in the sense here intended, are two totally different things. We mean something that shall render the particles so small as that they shall only be observable on being magnified, say, 50 diameters. Hardly any clays used in brickmaking are in bulk made of such small particles as this; there are a few, of which the best terra-cotta and porcelain are manufactured, however, but even these have to be very carefully prepared to exclude grosser foreign particles. From what we have said, it will be gathered that the terra-cotta and porcelain manufacturer is at the present time in a better position to judge of the work done in the kiln or oven than is the brickmaker. But that is simply a matter of education; the problems presented to the average brickmaker are rather more complicated than to the terra-cotta manufacturer, but they may be unravelled on sufficient application, as we hope to point out.

Even under the most favourable conditions, however—when the particles composing the mass require a ¼-inch objective for their elucidation—we find that the best burnt brick is largely made up of them in an unmelted condition. And we should be very sorry to get rid of them; for if they disappeared, the stony attributes of the brick would disappear also, and the general value of the substance would be deteriorated to such an extent that it would be unsaleable as a building material. The brick would nearest resemble a form of slag. All we now insist upon is that in brickmaking a chemical analysis is only useful up to a certain point, beyond which we must appeal to the microscope to aid us, and this in conjunction with as perfect a knowledge as possible as to the behaviour of earths of certain mineral composition when under the influence of high temperatures. In many instances, the value of the brick depends almost entirely on incapacity for fusion on the part of a large proportion of the minerals of which the brick is made. Possibly, a good all-round brick would be where the bulk of its mineral particles were infusible at the temperature employed, and when the remainder were fusible enough to partially run, so as to cement or agglutinate the infusible particles firmly together. In order to bring about such conditions artificially, or to arrive at them even approximately, we must know at least three things, viz.—(1) the nature of the mineral particles involved in the whole operation; (2) their behaviour under high temperatures; and (3) a knowledge of certain branches of metallurgical chemistry. Now, obviously, we cannot undertake to teach even the spirit of what is involved in these three desiderata in a small book like this; but we can, and shall, attempt to do something in that direction, and we must ask the reader’s indulgence to take for granted observations to be occasionally made, in the inevitable prospect of our not being able to explain them at sufficient length.

The following are the principal minerals found in clays used in brickmaking, together with their more important attributes from our point of view.

KAOLIN.

Pure clay is, theoretically, composed of this mineral alone, but pure clay does not exist in Nature, except as a mineralogical curiosity. What is generally called pure clay is a white, or light-grey plastic material, composed of kaolin with many other substances to a small degree, from which it frequently has to, as far as possible, be separated before being put to its highest uses in porcelain manufacture. Chemically, pure kaolin may be regarded as a hydrous silicate of alumina, viz.—silica = 46.3, alumina = 39.8, and water = 13.9. Under the microscope, in reflected light, it is seen to be made up of extremely minute, thin, six-sided plates, which are said (doubtfully) to crystallize in the rhombic system; though, when regarded with the naked eye, one would not suppose that it possessed a crystalline structure, as it appears to be an earthy, unctuous substance. It is commonly mixed with grains and small crystals and fragments of quartz, which mineral will presently be described. Being derived from the decomposition of felspars, the microscope reveals the fact that in addition to the six-sided plates alluded to, a great deal of opaque matter, as particles of mud, occurs in the substance universally known as kaolin. It is very difficult to satisfactorily state what this mud is; micro-chemically, its general character may be brought out. There is no doubt, however, that in converting the kaolin into china-ware, these particles are more active than the minute kaolin crystals in uniting with other substances to form a species of flux. The subject has been investigated to a very limited extent, but from the foregoing observations it will be seen that the proportion of amorphous mud particles to the minute crystals must be an important factor in determining the nature of the fluxing material, and of the quantity of this latter to be used. Correlatively, the fusing point can be determined in the same manner. For, in itself, kaolin is an infusible mineral, and before it can be made use of for brickmaking, terra-cotta, or any kindred purpose, it must be rendered artificially fusible by the addition of a fluxing substance. When, therefore, we learn that kaolin is being used for these purposes, we know, if used direct as it comes from the pit, that it must be impure from a mineralogical standpoint, or that it is being mixed with other substances. We say that kaolin is infusible (refractory); we mean at any temperature used in the industrial arts, including brickmaking. With the recent improvements in the electric furnace, the temperature generated is so high that practically any mineral substance may be melted; it is hard to speak of anything being infusible.

But the mineral matter called kaolin in ordinary clays, such as the brown and blue London Clay, the Oxford Clay, “brick-earths,” etc., has very little in common with the more or less pure china-clay. The microscope shows that in the vast majority of such clays scales of true kaolin are few and far between, that opaque mud particles are more frequent, and, above all, that pieces of highly decomposed felspar (called “kaolinised” matter) are present. Eliminating all other and foreign substances from the clay, the whole of what would commonly be called kaolin and kaolinised matter, taken together, is of very varied chemical composition, and might, indeed, be fusible in the ordinary sense of that term. From this, the reader will perceive that the term kaolin is very ambiguous and altogether too wide in its meaning. We think it highly desirable, therefore, to describe kaolin as a true mineral and not as a rock, reserving the term for the crystalline plates. The mud particles referred to we may call “kaolinised particles;” and the highly decomposed felspar “kaolinised matter.” To sum up the relative fusibility of these substances, per se, we should say that (1) kaolin crystals are practically infusible; (2) kaolinised particles are either fusible, partly fusible, or infusible, depending on the actual nature of the particles; and (3) that kaolinised matter may be difficultly fusible or infusible. A mixture of (1) and (2) may not be fusible, and could not be unless a great proportion of (2) of a fusible character, so as to form a flux, were present. The reasons for this will appear in considering the different kinds of felspar, next to be described.

FELSPAR.

This mineral, a very common constituent of nearly all clays and brick-earths is very variable in character, but may be separated into a number of mineral species, each of which possesses a definite structure and a more or less constant chemical composition. To show the range of variation, the following kinds of felspar, with their chemical composition, may be quoted:—[3]

Chemical Composition of Felspars.

Orthoclase felspar, in addition to the above, frequently has small proportions of lime, iron, magnesia and soda. Amongst other things it is an essential constituent of granite, and on the decomposition of that rock is the first mineral to become affected. When attacked in the open air by rain and the ordinary agents of denudation, granite ultimately gives way by the dissolution of the felspar, and on being removed, the felspathic matter may accumulate in convenient situations to form kaolin. If we now compare the chemical constitution of orthoclase felspar with that of kaolin as previously given, we notice that the potash has disappeared in the decomposing process; it has been dissolved and taken away by rivulets, and the like, or washed by rain direct into the sea. We also observe that there has been a re-distribution, so to speak, of the relative proportions of silica and alumina—following well-known laws.

Of the remaining felspars the commonest for our purposes is oligoclase, a mineral found in nearly all British “granites” in a greater or less degree. That contains a higher percentage of alumina than orthoclase, and there is a fair proportion of soda and little lime, but much less potash. The lime-soda felspar, labradorite, and its near ally, anorthite, are not often met with in a recognisable form in clays. If present, they are generally as “kaolinised matter,” too highly decomposed to exhibit their characteristic optical properties.

It is pretty generally stated, and too often assumed by some, that pure china-clay is derived from the direct decomposition of rocks containing “orthoclase” felspar. Yet, this cannot really be so, if we reflect on the mineral composition of many of the rocks, which, obviously, have yielded the china-clays in question. Take the china-clays of Devon and Cornwall; they have undoubtedly been derived from the “granites” of those counties. To some extent, as previously remarked, the orthoclase is attacked, and provides the material of which china-clay is made. But in the “West of England,” we have yet to learn that some of the other felspars are not also involved in the process. If we examine a fresh piece of granite from the flanks of Dartmoor, or from the neighbourhood of Liskeard, or St. Austell, we find no difficulty in recognising a fair proportion of triclinic felspar (one or more of those mentioned in the table except orthoclase) in it. There is a difference in the composition (and therefore the commercial applicability) of a china-clay derived from a rock containing orthoclase alone, and one from a rock having orthoclase and one or more triclinic felspars in addition. The latter minerals are more easily decomposed than orthoclase, especially the lime and lime-soda varieties. We should not have raised this point only that, by reason of the granites being to some extent mechanically as well as chemically decomposed, a large proportion of “kaolinised particles” and “kaolinised matter” is introduced into certain china-clays, which render them different in their behaviour under intense heat from those china-clays in which orthoclase alone has been principally concerned. In other words, great practical advantages accrue from an accurate knowledge of the constitution and origin of the china-clays in question. Two clays of the same chemical composition often behave in a different manner in the kiln; the cause of this is frequently to be found in the prevalence of “mechanical fragments” of felspar in one of the clays; and the absence of these, but the presence of “kaolinised particles” of the same chemical composition, in the other.

Another point to which we may draw attention is the erroneous supposition that granites which have yielded china-clay have in all instances been reduced to the condition in which we now find them by the action of atmospheric agents of denudation alone. Granites, as a matter of fact, yield very slowly to the action of the atmosphere, and taken as a whole no building stone is as durable as they. How comes it, then, that they have decomposed to such an extent as to have formed extensive deposits of china-clay in a very short space of time, geologically? We think the answer is to be sought, at any rate in some instances, in the alteration the rock as a whole has undergone in certain situations, whereby it became more easily decomposable. Take the rotten china-stone of the neighbourhood of St. Austell, for example. In that material we clearly see a stone from which the “life” has been sapped, and instead of a bright, sparkling, porphyritic granite, as it once was, we now notice only a ghost of its former self. The large orthoclase felspars may be seen in it as skeletons, the mica is reduced to mere iron-stains (when present at all), whilst the quartz is also slightly affected. This altered and comparatively rotten material (although sometimes hard enough to be used as building stone) extends to an enormous depth from the surface; it has not been bottomed in some parts of the district. Such an extensive transformation could not possibly be due to ordinary agents of denudation which do their work at and near the surface of the rock only. It seems to arise from an enormous regional alteration, acting underground to an unfathomable depth, and which may not be unconnected with the mineral veins so common in, and in the immediate vicinity of the workings.[4]

Yet another thing to be remembered is that, under certain conditions, as near St. Austell, china-clay has been formed in situ, and has therefore not been deposited by the action of running water, as have the majority of china and other clays. Mr. Collins remarks that this china-clay is very irregular in its occurrence. It seems to be formed of various granite masses decomposed in place; it often occupies considerable surface areas, and extends to a depth unknown. He remarks that at Beam mine, and also at Rocks mine, both near St. Austell, china-clay was found to a depth considerably exceeding 60 fathoms from the surface. This china-clay, in its natural condition, is very much the same as china-stone; but the decomposition has proceeded further, the felspar being completely changed into clay; and nothing more is necessary for extracting the clay than the disintegration of the whole mass by a stream of water directed upon it, when the clay is carried away in suspension and collected at convenient spots. Thus there is every gradation between the true crystalline orthoclase and triclinic felspars, through china-stone into china-clay formed in situ, so into china-clay deposited from water by natural or artificial means, and into a pure clay containing a large proportion of kaolin crystals, “kaolinised particles” and “kaolinised matter.” But although we can state that much, a great deal yet remains to be done in connecting mineral structure with chemical composition of the purer clays, and in defining the various grades scientifically, in order that full advantage may be derived from them in a commercial sense.

CHAPTER V.
MINERALS: THEIR BEHAVIOUR IN THE KILN.

THE SILICA GROUP.

Silica, the oxide of silicon, is found in brickmaking clays principally in two conditions when not combined with other substances: in one of these the free silica may be crystalline, when it is known as quartz; in the other it may be hard, but not crystalline, as flint. We may consider these in order.

Quartz.—When pure this mineral is perfectly white and transparent, like ordinary window glass. It is exceedingly hard, and this property is of much service as enabling us by the most elementary examination to distinguish it from certain other minerals, which it is not unlike at first sight. One of the latter is calcite, a crystalline form of carbonate of lime, also white and transparent. Quartz and calcite behave in a very different manner in the kiln, and as we shall see, they are both rather common constituents of brick-earth. The difference in hardness may easily be ascertained by the point of a good steel knife; the steel will not scratch the quartz, but it will, easily, the calcite.

When it has plenty of room wherein to crystallise, and is not hemmed in, as it were, by other hard crystalline matter, quartz often forms beautiful six-sided prisms surmounted by a six-sided pyramid, and, rarely, pyramids are found at both ends of a prism. There are no lines, or “planes of cleavage,” to interfere with the transparency, either in the extremely minute forms of the mineral as investigated by the microscope, or in the gigantic crystals occasionally found. Regular crystals of quartz, although by no means rare in Nature, are seldom met with entire in brick-earths. The most common form of the mineral is in irregular aggregates with other minerals, as in the rock granite, which is composed essentially, as previously mentioned, of quartz, felspar, and mica. We have traced the history of the felspar on the decomposition of that rock, and it may now be said that on complete disintegration of the granite a great part of the quartz present is simply resolved into fragments and dealt with by rain and other transporting agents. For quartz is practically imperishable; it is almost proof against the deleterious acids in the atmosphere, which so readily attack many other common minerals. In dealing with it, all Nature can do (at least at the surface of the earth) is to carry the small quartz grains and pieces about from place to place; She can, and does, in this process, reduce the quartzose fragments by causing them to continually knock against each other and against other mineral fragments and masses until the grains and pieces find a resting place; She may put them in a mill and grind them to powder, but the quartz is still there.

Another manner in which quartz occurs in Nature is as filling cracks in rocks, but this is comparatively unimportant for our present purposes. The purest quartz is known as rock crystal; but by far the commonest kinds of the mineral are impure; they may contain iron, schorl (a black needle-like crystal), and many other minerals. One of the most interesting points about it, and which undoubtedly in certain cases is of importance to the brick manufacturer as modifying its melting properties, is the presence of myriads of extremely minute so-called cavities, generally filled (or nearly filled) by liquids of different kinds, the precise nature of which is not as well-known as it might be, though in some instances it has been determined with tolerable certainty. In some cases these inclusions are so numerous as to obliterate the transparency of the quartz crystal, causing it to present a frosted appearance. The fluids in these cavities may have beautiful little crystals of other minerals, such as salt, floating about—but it must be remembered that we are referring to something infinitely little. These slight differences in the constitution of minerals, however, have their influence in the kiln. For instance, although the fluid present is usually water, that often contains carbon dioxide, which acts as a species of flux to the quartz when present in sufficient quantity.

In reference to the second form of silica present in brick-earths, flint, that is of precisely the same chemical composition as quartz, only that it is not crystalline, nor transparent, though thin pieces of flint are translucent. Flint is by no means as common in Nature as quartz; it is very hard, but brittle, and breaks with what is termed a conchoidal fracture, from the fact that the fractured surface frequently resembles the external appearance of the shell of a bivalve mollusc. It occurs in a variety of ways; (1) often as hard lumps or nodules running along in fairly regular layers in limestone rocks such as chalk, and (2) occasionally filling up cracks or joints in such rocks. It is hard to describe its origin in a few words, and we shall not attempt it; all that need be noted is that it is frequently full of the remains of extinct organisms of small size, which may, or may not, constitute an impurity depending on the particular organism and its present condition. When flint contains a fair proportion of iron it is called chert—an extremely common constituent of brick-earths in some localities—though that term refers to other rocks, such for instance, as those made up almost exclusively of the siliceous spicules (hard parts made of silica) of fossil sponges.

A more or less crystalline kind of silica is found, forming the skeletons of minute aquatic plants, and these accumulating to some depth, constitute the basis of such materials as Kieselguhr and the diatom earth of the Isle of Skye, both of which, especially the former, are used for making firebricks.

There is very little to be said concerning the behaviour of free silica—quartz and flint—in the kiln. It is infusible except at higher temperatures than are employed by the brickmaker. But, as we have already remarked, the impurities often present in the minerals form a species of flux which naturally brings them into the range of fusible substances, though even then the temperature required is far beyond what is usually attained in the majority of brickyards, though it might be frequently arrived at in the manufacture of certain fire-bricks. For all ordinary purposes, therefore, quartz and flint may be regarded as infusible. In presence of much lime, iron, or similar substances, however, both of them are readily melted, and it is part of the science of brickmaking to know exactly how much lime, &c., to add to yield the best results. Many brick-earths contain large quantities of the calcareous and ferruginous substances alluded to, and are then capable of being made into bricks direct, without any addition. But although such natural brickmaking earths are frequently employed by the manufacturer, nearly all of them could be made to yield a better brick by a little artificial mixing. We must keep urging this point; there is room for great improvement all round.

As with the majority of comparatively refractory substances, the size of the grains and pieces of quartz and flint makes a difference in their readiness to become fusible. The larger the grain the more difficult it is to break down; fusion commences at the outside of a quartz grain, the centre of which may at the same time be comparatively unaffected. By arresting the fusing process, the microscope shows the outside of the grain to have become softened (so much so as to affect its doubly refracting properties), whilst the innermost parts still retain their usual optical characters.

MICA.

The different varieties of mica are important as rock-forming minerals, but they are not as often met with in brick-earths as is generally supposed, except in insignificant quantity. Some of the purest clays, however, contain a great deal of mica, derived almost directly from the destruction of granite. The two commonest varieties of the mineral are biotite and muscovite.

Biotite Mica.—This mineral, usually known as ferro-magnesian mica, is composed of silicates of magnesia, alumina, iron, and alkalies in variable proportions. It occurs as six-sided plates or irregular scales, usually of a bronze-black colour. Biotite weathers with comparative facility, hence the reason why it is not more commonly met with in brown and other impure clays.

Muscovite Mica.—This is sometimes called potash- or alumino-alkaline mica, composed of the silicates of alumina, alkalies, iron, and magnesia; the proportion of silica ranges from 45 to 50 per cent. It may usually be distinguished at sight from biotite by its silvery white or light brown colour. When large enough, both the micas mentioned may be split up into thin plates, muscovite yielding large transparent sheets. Compared with all other constituents of brick-earth, the micas are bright and of semi-metallic lustre. Muscovite is more durable than biotite, and is much more frequently met with in brick-earths, especially in the sandy varieties.

The influence of mica in the kiln is not of much importance in ordinary brickmaking; in general its alkaline character renders it fusible, though a high temperature is necessary at all times to effect that. In china-clay mica is regarded as a nuisance, and in breaking down the material it is separated in the washing process by running water, the mineral collecting in depressions or basins, called “micas.” When muscovite contains much fluorine, as it frequently does, it is very undesirable in clays for high-class purposes. At the best of times the proportion of iron in mica is sufficient to mar the quality of the otherwise most excellent clays. In the kiln, or porcelain furnace, the presence of mica (more particularly biotite) is apt to create yellow and brown specks, or a species of mottling. It is highly satisfactory, therefore, to note that these little shiny flakes may be easily floated off by a moderate amount of care in washing, and thus separated from the other constituents of the clay.

IRON.

Except in regard to white kaolin clays, nearly all earths used in brickmaking contain more or less iron, which is usually present as protoxide in many mineral constituents. The colouring matter of clays is generally iron in some form, and blue clays weather into brown by the alteration of that mineral. It is unnecessary for us to consider the various minerals of the iron group; all we need do is to state the mode of occurrence of iron oxides in clays and earths, to consider a variety known as iron-pyrite, and the general effects of ferruginous minerals in the kiln.

Iron may occur in clays simply as a stain, when it is usually not in large quantity, or it may occur combined with some mineral or minerals present—as for instance certain felspars and micas. The brown, yellow, or blue appearance of the clay is due to it. In loam it may be found also as a species of ochreous earth, and in thin bedded loams (as the upper part of the Woolwich and Reading series of the London basin) each layer frequently varies in the proportion of iron present. In the more arenaceous parts of these loamy deposits, little grains of iron sometimes make their appearance, as also in certain sands employed in brickmaking; on careful examination, however, many of these grains are found to be other mineral substances coated with iron. Certain horizons in what are known as the Jurassic rocks contain great quantities of ferruginous matter in little pellets.

Iron, in large proportion, acts as a flux to other constituents when the brick-earth is subjected to great heat in the kiln, and on that account must be carefully watched. But, to the average brickmaker, the ferruginous constituent is far more interesting as a colouring medium. At a later stage we shall have something to say concerning the colouring of bricks, &c., but it may now be remarked that red bricks, in practically all cases, owe their colour to the effects of firing on iron. It is a great mistake to imagine, however, that a large percentage of iron in a clay will necessarily produce a good red tint. In the first place, a great deal depends on the way the clay has been mixed or prepared; and in the second, the method of burning and the temperature employed, taken in conjunction with the general composition of the earth, are all important. This much may be said, however, that without the iron (or some mineral colouring matter possessing similar properties in the kiln) a red brick would not result. An even colour is the effect of thorough and homogeneous incorporation of the iron with the brick-earth; that may have been brought about by natural processes, but it is most frequently obtained in the careful preparation and mixing of the clays. A very essential point is that the earths must be of such a character as to withstand the requisite heat in the kiln without becoming vitreous, or twisting or warping. It must not be forgotten that a certain proportion of the iron, under great temperatures, may be carried away out of the kiln in union with other things, in the form of vapour. To successfully treat a raw earth, so that all these points may be taken into account, and to produce a thoroughly uniform red brick, that shall not vary in tint from kiln to kiln, is a matter requiring considerable skill and attention, though fairly good bricks of that character have been produced by sheer accident in burning natural earths fairly rich in thoroughly disseminated iron oxides.

Two minerals commonly met with in earths used for brickmaking are pyrite and marcasite, both of which are of the same chemical composition, namely, iron disulphide. We may first consider them separately, for they are of great importance to the brickmaker.

Iron pyrite occurs as regular cubic crystals, or irregular streaks, or as nodules or lumps; in clay, the last-mentioned is its commonest form. It is a good petrifying medium, so that it is frequently associated with organic remains, as is exemplified in almost any yard where stiff clay is being worked. The nodules, on being broken open, ordinarily exhibit a radiating structure of brassy lustre and extremely beautiful appearance, though often marred by brown iron stains due to decomposition of the mineral. In the refuse of slates, now so largely used in several parts of the world for brickmaking, pyrite is most frequently found as fine cubic crystals of a durable nature.

Marcasite, on the other hand, crystallizes in a different manner (in the rhombic system of mineralogists), but is chiefly found in fibrous masses or dirty-brown nodules, the last-mentioned being common in clays. When bright it is paler in tint than pyrite, though this is not a constant character. It occurs abundantly in almost all sedimentary rocks diffused as minute particles, but sometimes in irregular layers. Sir Archibald Geikie states[5] that this form of the sulphide is especially characteristic of stratified rocks, and more particularly of those of Secondary and Tertiary age. That it is not abundant in Primary rocks is not to be wondered at when we consider its liability to rapid decomposition; indeed, for it to be preserved at all it must be well shielded from atmospheric agents by Nature. Exposure even for a short time to the air causes it to become brown, free sulphuric acid is produced, which may attack surrounding minerals, sometimes at once forming sulphates, at other times decomposing aluminous silicates and dissolving them in considerable quantity. It plays even a larger part than pyrite as a petrifying medium, at any rate in the younger rocks. Both pyrite and marcasite are abundant in many other rocks than those of special interest to the brickmaker; the former, in fact, is almost universal in its occurrence.

It will be convenient to consider the behaviour of these two minerals in the kiln together, as the difference between them from that point of view is practically nil. Under the action of the intense heat met with there, they become partially decomposed; oxide of iron and basic sulphides of iron remain. When, at a subsequent period, bricks containing these substances are exposed to the action of the weather, oxidation takes place, sulphate of iron and sometimes of lime are formed, which on crystallizing expand with considerable force and split or crack the brick. From this it is evident that sulphide of iron in any form is not to be tolerated in brick manufacture, and if the earth used in the first place contains much, it must be removed in the preparing process. If permitted to remain, it is impossible to obtain either a durable, or a good coloured brick.

CHAPTER VI.
MINERALS: THEIR BEHAVIOUR IN THE KILN (continued).

CALCITE, ARAGONITE, &c.

Carbonate of lime may occur in a crystalline form, or as earthy substances, and many varieties of it are found in clays used by the brickmaker. The commonest are calcite, aragonite, and a white earth.

Calcite, known also as calc-spar, crystallises in the hexagonal system, though true hexagons are not very common. It occurs principally as rhombohedra and scalenohedra, with variations therefrom; also fibrous, lamellar, granular, compact, nodular, and stalactitic. When pure, calcite is colourless and usually transparent, but when mixed with iron or other mineral colouring matter it commonly assumes yellow and brown tints.

Aragonite is also a crystalline form of carbonate of lime, but is by no means as common in Nature as calcite. It crystallises in the rhombic system, which assists the mineralogist to distinguish it from the last-mentioned mineral, from which it differs also in being harder and of higher specific gravity. Aragonite may occur as globular masses, or as incrusting other substances, or in the stalactitic form. It is sometimes white, but more often yellowish, or grey, and it is not, commonly, as transparent as calcite, whilst it often possesses one to two per cent. of carbonate of strontia, or other impurity.

It is generally stated that carbonate of lime, when deposited from cold solutions, crystallizes in hexagonal (calcite), and when from warm solutions, in rhombic (aragonite) forms. No doubt, on the whole, that is the case; but we ought not to forget that many marine organisms make their hard parts of aragonite, which, under the circumstances, is certainly not obtained from warm solutions. These crystalline forms of carbonate of lime are both of them found in fossil shells and the like in clays, and in not a few instances the calcareous constituent found in the brick-earth is present almost exclusively in the fossils, which are ground up with the rest in preparing the material for the moulding machine.

When present as hard crystalline lumps or pebbles, they have been derived from the destruction of limestones, and are then the greatest nuisance imaginable to the brickmaker and the most dangerous constituent at the same time. With proper machinery these hard lumps may be ground down to fine particles, but they are even then only to be admitted into the earth on sufferance. The best plan, without doubt, is to remove them altogether from the raw earth. They are commonly met with in what the geologist calls “boulder clay”—a deposit owing its origin to glaciers and icebergs. Very often the pebbles alluded to are not crystalline, but of an earthy character, as is the case when made of chalk. In the semi-dry process of manufacture, it is next to impossible to incorporate the ground-up particles of carbonate lime sufficiently well to result in the production of such a homogeneous earth as is desirable for making a first-class brick.

In sandy clays or loams, and in a few stiff clays used for brickmaking, certain remarkable concretions called “race” are found, the deleterious properties whereof are so well known to the average brickmaker that he carefully avoids the particular strata in which they occur. It is fortunate that these concretions have a habit of being confined to narrow limits along definite horizons in the brickyard section, so that they may be readily discarded in working. But that is not always the case, and little nodules of “race” are usually more or less frequent also in the beds above and below the horizons referred to. They are composed wholly of carbonate of lime, and their general effect in the kiln, and afterwards, will presently be explained. Other forms of concretions are known as “septaria,”—tabular or rounded masses of argillaceous limestone found in practically all stiff clays. These are often of enormous size, and are disposed in regular lines which the field geologist takes to indicate bedding planes in the clay—otherwise often very difficult to make out. In certain stiff clays little pellets of the same substance are found. The larger septaria have commonly been cracked in various directions, the fissures being subsequently filled with calcite.

Coprolites are impure varieties of phosphate of lime, and the term should, properly speaking, be restricted to a substance of organic origin,—the fossilised excrement of animals. But the name is now loosely employed to designate phosphatic concretions in general, such as are commonly found in stiff clays, in certain “greensands,” and in other sedimentary deposits. The dark brown phosphate of lime has formed on and often completely envelopes many fossils; in certain cases it has in fact been utilised as a petrifying medium, in which form it ordinarily occurs in the thick black clays of Peterborough, Cambridge, the gault of Kent, Surrey, etc.

Summing up the effects of carbonates and other kinds of lime in the kiln, it may be at once said that when present in any other form than as extremely minute particles, they are distinctly to be avoided. The small pellets and large pebbles especially are to be avoided, for the following reasons. Carbonate of lime is made up of lime and carbonic acid; if a lump of this be subjected to great heat and thus calcined, the carbonic acid is driven off, escaping by means of flues, the open chimney, or kiln. The product is lime pure and simple—ordinary builders’ lime. Everyone knows that on the addition of water builders’ lime becomes “slacked,” and eventually, after a fashion, “sets.” Precisely the same thing occurs in the brick-kiln. The raw brick is often composed of pieces of chalk or other limestone, in limestone districts and in areas where boulder clays are largely employed for brickmaking. On being subjected to the heat of the kiln these pieces are promptly reduced to the condition of lime. During the process of conversion considerable expansion takes place, and subsequently contraction, leading to the formation of cracks radiating from the fragments of limestone, the homogeneity of the bricks being at once destroyed. Apart from this, when placed in the open air the lime becomes slacked, and the quality of the brick is seriously impaired.

Lime is a highly refractory substance, strongly basic in character, and forms fusible compounds with silica and other acid bodies. It is, therefore, useful as a flux in many earths used in brickmaking, being added to them expressly for that purpose, to the general improvement of the brick. The celebrated Dinas bricks, for instance, are composed of a highly refractory earth containing about 97 per cent. silica, the remainder being lime, oxide of iron, alumina, alkali and water. To render this material fusible and so as to make refractory bricks, from 1 to 3 per cent. of lime is added.

But what we more particularly desire to draw the reader’s attention to at the present stage, is not the employment of lime in making fire-bricks so much as its mixture with ordinary brick-earth, as in the manufacture of malm bricks. Sometimes the mixture has been effected by Nature, as is the case with true marls; but the brickmaker does not care so much for these, as without considerable and expensive artificial assistance they do not often make readily saleable bricks. The common practice is, briefly, to grind chalk or similar earthy limestone in the wet state, and then to introduce it to the brick-earth with which it is thoroughly incorporated; and there are many ways of doing this, which we shall not attempt to describe now. The object of adding chalk to the brick-earth is twofold; in the first place it assists in diminishing the contraction of the brick on drying, i.e., before burning; and secondly, it acts as a flux in the kiln by combining with the free silica, or the silicates, in the earth. Undoubtedly the second is, theoretically, its chief function; but its beneficial effects in that direction are largely marred by insufficient burning, whereby a large proportion of the chalk is not actively engaged, as may be seen on examining the majority of malm bricks with the microscope. Indeed, the eagerness to save fuel, and to turn out the bricks as rapidly as possible, often leads to the chalk particles being utterly useless. And, if we may judge from conversations with several brickmakers, it would seem that the real reason why the limestone is used at all is unknown to them, except that it produces bricks of a saleable colour. This question of colour is the all-predominating one with most malm brickmakers.

We said just now that the fragments of limestone in the raw brick are reduced to lime on being burnt; some of the latter, however, as may be anticipated from our subsequent remarks, is engaged in forming a flux wherever possible in the immediate neighbourhood of such fragments: it is the “kernel” that is left which becomes “slacked,” and weakens the brick. The object of utilising the smallest particles only of the carbonate of lime is thus obvious; and if it were possible to use ordinary builders’ lime instead of carbonate of lime, the result would be better still. The difficulty in utilising builders’ lime is, of course, its certainty of slacking during the preparation of the brick-earth with which it would have to be thoroughly incorporated.

SELENITE.

The “petrified water” of the brickmaker. It is a crystalline form of gypsum—a hydrous sulphate of lime, occurring in large quantities in the commonest clays used in brickmaking. Large and beautiful crystals, some of them radiating from a central point, are found in the London Clay, Kimeridge Clay, Oxford Clay, &c. By expelling the water from selenite, or gypsum, plaster of Paris may be prepared. In the kiln, therefore, it is important that this constituent be as finely ground as possible, so as to localise the effects of the anhydrous sulphate on being moistened subsequently. In hard burnt bricks, no doubt, a great deal of it is effectively used as a flux to other constituents of the clay; but in by far the larger quantity of bricks this sulphate is reduced to fine powdery particles easily picked out as being softer and lighter in tint than the remaining constituents. The weather-resisting qualities of the brick are naturally, not improved when much baked selenite is present; and the colour of the whole is apt to become variegated—that is, in a fairly soft brick.

DOLOMITE.

Dolomite is, chemically, composed of the carbonates of lime and magnesia in about equal proportions. It is found as rhombohedral crystals, the faces of which are often curved; also in granular and massive conditions. Its prevailing colour is light yellow both in crystals and rock masses, but, as with most other minerals, impurities occasionally make it assume other tints, principally red and green. Carbonate of iron is frequently present, sometimes to such an extent as to entirely alter the character of the substance. As separate crystals dolomite has very little interest for us, though rarely it may take the place of calcite or aragonite in the fossils of brick-earths and clays. But in its massive condition, as magnesian limestone, it is of increasing importance to the brickmaker. For many years it has been utilised in the manufacture of basic bricks, though at the present moment the market in these materials is attentively looking at the possibilities of the next mineral to be described.

MAGNESITE.

Magnesite is pure carbonate of magnesia—that is, magnesia = 47.6, and carbonic acid 52.4 per cent. It usually occurs massive or fibrous, but sometimes granular, and its fine rhombohedral crystals are well known. Like dolomite, its prevailing tint is yellow or light brown, but, when very pure, is as white as snow. It is usually associated with serpentine rocks. In the kiln it is highly refractory, and behaves very much in the same way as lime—forming fusible compounds with silica and silicates. For the higher grades of basic bricks it is at this moment largely exploited in the few localities where it occurs in paying quantities. A few years since, investigation to determine the best basic refractory material was actively prosecuted in Germany, and magnesia, preheated at the highest white heat, was awarded the palm. Magnesite, when calcined, yields magnesia, which, however, still contains the impurities that might have been present in the raw material. An average percentage composition of the magnesite of commerce shows it to contain magnesia 45, carbonic acid 50, lime 1.5, protoxide of iron 1.6, the remainder being silica, alumina, and protoxide of manganese. The presence of silica in magnesite is an objection, because it is liable to have a fluxing effect at high temperatures.

Magnesite has been found in paying quantities in California, Styria, and recently in Greece. In Eubœa, in the last-mentioned country, the mineral occurs in lodes which, near Krimasi, are worked on two levels 30 to 40 feet from the top, and dipping at an angle of about 70 degrees. The general average of the lode gives 88 per cent. of carbonate of magnesia, and the substance is peculiarly suitable for the manufacture of basic bricks. A novelty with the raw material is that the proprietors sell either by guaranteed degree, or degree of analysis, the former being 95 per cent. of pure magnesia, whilst the latter often gives as much as 97.8 per cent. In inferior grades the principal increase is in the proportion of silica.

SALT.

Chloride of sodium, or common salt, is present in many natural clays, especially (in England) in that formation known to geologists as the Trias, developed largely in Cheshire. The influence of a salt-bearing bed is, naturally, not confined to the immediate vicinity of the formation; salt being so readily soluble in water, it comes forth from the rocks in springs, which, flowing over loams and other similar absorbent earths, impart a saline character to them. In this manner otherwise useful earths for brickmaking are rendered absolutely unfit for the purpose. Salt is one of the most powerful fluxes known; when mixed even in very small quantities with clay it becomes impossible to make a good brick of the substance. But we must recur to this matter at a later period in another connection. The fluxing property is sometimes taken advantage of by mixing salt with sand in moulding, or in employing a sand already saline, as when dredged from the sea, or obtained between tide-marks. A species of glaze is produced on the brick by the action of such moulding sand.

We may ignore the presence of a number of minerals such as rutile, augite, and hornblende in brick-earths, as they only exist therein in such small proportion, and have no appreciable effect in the kiln.

CHAPTER VII.
THE CHEMISTRY OF BRICK-EARTHS.

Introduction: THE BLOWPIPE.

It is not our intention to write an elementary treatise on chemistry; but we know it is the custom for brickmakers to have chemical analyses of their raw earths made, and we are aware also that the precise meaning to be attached to these analyses is very little understood. Our principal aim in introducing this subject, then, is to interpret, in an elementary manner, certain typical analyses of earths and substances used in brickmaking; but before doing so we shall explain some easy methods of examining earths by means of the blowpipe, which will not merely give some insight into their chemical constitution, but will afford the intelligent brickmaker a means of investigation which he can himself put into practice.

The results of a chemical analysis of a compound earth, as ordinarily used by the brickmaker, widely differ from those obtained by a mineralogical or petrological examination. The petrologist views the earth as a mineral aggregate, the constituents of which may be ascertained on appeal to a properly-constructed microscope—that is, in the majority of instances. By noting the relative proportions of the different minerals, he is enabled to state, with approximate accuracy, what is the ultimate chemical composition of the whole. From this it would appear that a rough chemical analysis could be drawn up by the petrologist without having recourse to the ordinary methods of chemical investigation. And in a limited sense that is true. But we should not lose sight of the fact that there is, in too many cases, an amorphous residuum in earths, the nature whereof the microscope is powerless to reveal. It is upon this remnant that the chemist should direct his most careful attention.

The mineralogist also can give a shrewd idea of the chemical composition of a brick-earth by using a blowpipe and accessories. This, in fact, may be regarded as a chemical means of investigation; but it possesses this serious drawback, viz., the blowpipe only yields a qualitative, and not a quantitative analysis. In other words, it can tell us something concerning chemical compounds present in an earth, but rarely informs us as to the relative proportions of them. Even this, however, is of great service in many instances, though it does not possess the value of a quantitative analysis. For example, we have stated previously that certain ingredients are very undesirable in a brick-earth, even in minute quantities; and that fact becomes of increased value if we extend the field to earths used in terra-cotta, and china and porcelain manufacture. Now, the blowpipe is a handy instrument; it may be carried about by the prospector with its usual accessories, and occupies but little space. Suppose he discovers a bed of white earth which he believes to be good china-clay; he can prove that fact, or at least obtain a great deal of information to that end, by the mere use of that useful little instrument. Knowing, for example, that fluorine is an undesirable constituent in such a clay for many high-class purposes, he might test first of all for that; iron, perhaps, may come next, and so in a few minutes he is enabled to arrive at some valuable particulars that would take much longer to obtain by chemistry in the wet way.

It will be profitable, therefore, for us to briefly describe the blowpipe and the most common of its accessories, stating results obtained in dealing with substances frequently met with in brick-earths. With but little practice anyone can use the instrument, though, as with most other methods of scientific investigation, it requires expert knowledge to yield really excellent results. The simple minerals and compounds to which we shall direct attention may be detected with the greatest ease.

The essential constituents of a blowpipe outfit are as follow:—

1. Blowpipe.
2. Lamp.
3. Platinum-pointed forceps.
4. Platinum wire.
5. Charcoal.
6. Glass tubes.
7. Chemical reagents.
8. Miscellaneous articles.

Fig. 5.—Blowpipes.

1. The Blowpipe.—Common forms of blowpipe are shown in [fig. 5]. A may be described as follows. It consists of three separate parts: a tube a b having a mouthpiece; an air chamber c to retain moisture caused by the breath of the person blowing; and a side tube d ending in a platinum-tipped jet. Another form of blowpipe, which, however, does not differ essentially from that just alluded to, is shown in [fig. 5], B. It is not absolutely necessary to have the jet made of or tipped with platinum, though certain examinations with the instrument are facilitated by the use of such a tip. An essential point is, that the hole in the jet should be of proper size, usually about 0.4 mm. The trumpet-shaped mouthpiece shown in the diagram may be dispensed with.

Fig. 6.—Blowpipe Lamp, &c.

2. The Lamp, or Candle.—A convenient form of lamp is a Bunsen gas-burner furnished with a special jet ([fig. 6], A). For certain purposes, however, this flame cannot be employed, as when testing a substance for sulphur, as coal-gas frequently contains sufficient sulphur to vitiate results. Moreover, in country districts and in the field coal-gas is not always procurable. A convenient form of lamp, though rather too large for transporting purposes, is known as Berzelius’ blowpipe lamp. This, as improved by Plattner, is shown in [fig. 6] B. This consists of an oil vessel on a stand provided with two openings closed with screw-caps, the one opening being used for charging the lamp with oil, the other being fitted with a burner bearing a flat wick. The lamp may be adjusted to any required height on the stand by means of a screw. Olive oil, or refined rape oil, is usually burnt. A spirit lamp with a flat wick is sometimes used. In countries where neither coal-gas, alcohol, nor oil are readily available, the prospector may use a small grease lamp. This consists of a cylindrical box of thin metal having a wick-holder soldered on one side, through which a flattened wick is drawn. The box may then be filled with grease, solid paraffin, old candle-ends, or fat of similar description. Professor Cole describes[6] it as follows:—When brought into use the wick is lighted, and the flame directed with the blowpipe upon the surface of the solid tallow or fat, until this is melted to a depth of about a quarter of an inch. The lamp will then become hot enough during use for a continuous supply to be maintained; but it is still better to hold the lamp with the pliers over a spirit lamp until all the contents become fluid. When about half or three-quarters empty, it is well to drop in extra lumps of fuel—a single candle-end or so—during use, and this additional material becomes melted up slowly with the rest. The wick must be freely supplied with fluid fuel, or it will char and waste away. If the lamp is kept sufficiently hot, the wick will not require raising during a day’s work; but it can be easily thrust up with a knife point after the flame has been at work a few minutes. A cylindrical cap fits down upon the lamp when put aside. For many ordinary purposes a good carriage-candle may be employed to give a blowpipe flame, but candles have the disadvantage of not remaining at a constant level—an important point when one is comfortably at work.

3. Platinum-pointed Forceps.—At least one pair of forceps is needed, and it should preferably be made of steel, nickel-plated to prevent rusting. One end has platinum points self-closing by means of a spring, so that the piece of mineral to be heated, placed between them, may be firmly supported. At the other end are other forceps of ordinary pattern for picking up small fragments; this end, however, should never be placed in the flame. A pair of common self-closing forceps might also be at hand for holding test-tubes, etc., in the flame.

4. Platinum Wire.—A few inches of thin platinum wire are indispensable, and lengths of an inch or so may be fixed into suitable handles. A convenient method is to have a small glass rod for a handle, and by fusing the tip of one end of the rod the glass may readily be made to hold the piece of wire. Pieces of platinum foil are useful, also, as will presently be seen.

5. Charcoal.—The outfit should comprise several pieces of charcoal, and a convenient form for each piece is a circular disc about an inch in diameter, flat at the top and convex beneath. Long prisms of the same material, square in section, are occasionally required; these may be up to 6 inches, or so, in length.

6. Glass Tubes.—These should be of hard glass, small, of several diameters, the bore being large enough to place fragments of minerals or earthy substances within. Closed tubes, such as test-tubes, are always requisite.

7. Chemical Reagents.—These are, for the most part, used as fluxes, and those most commonly employed are borax (sodium tetraborate), soda (sodium carbonate), and salt of phosphorus or microcosmic salt (phosphate of soda and ammonia). Small quantities of potassium bisulphate (in a glass bottle), as also small bottles of hydrochloric, nitric, and sulphuric acids, and a solution of cobalt nitrate, are also useful in certain cases. It is hardly necessary to remark that the chemicals employed must be of the highest degree of purity.

8. Miscellaneous Articles.—Strips of test paper, both turmeric and blue litmus, a small hammer, a steel anvil about an inch cube, a bar magnet, a pair of cutting pliers, a three-cornered file, and a few small watch-glasses are very desirable, though not absolutely essential.

The reader, on glancing at the foregoing formidable list of articles, may possibly imagine that some considerable outlay is requisite, and that they must occupy much space. But that is not the case. An ordinary blowpipe, a grease lamp, a small spirit lamp, and all the articles mentioned in paragraphs 3 to 8, both inclusive, occupy but a small space. They may be packed in a box specially fitted, and one in the writer’s possession, containing all of them, measures only 10 inches by 5 inches by 3¼ inches, and is less than 3 lbs. in weight.

Now, as to the use of these various things. First of all, let us examine the flame, as produced by a candle, which is typical of flames obtained by other means described, except the Bunsen lamp. A candle flame (see [fig. 7]) consists of the following parts:—

1. A dark core (a), which contains the gaseous products of decomposition given off by the melted tallow drawn up by the wick.

2. A highly luminous zone (b), in which only partial burning of the combustible gases takes place. In this, oxygen from the air combines chiefly with the combustible hydrogen, whilst the carbon is separated in a highly heated state, which causes the luminosity.

3. An outer mantle of blue tint (c), where the oxygen of the air is always present in excess, so that the separated carbon is here burnt. The highest temperature is found in this part of the flame.

Fig. 7.—Candle and Gas Flames.

Technically, the outermost zone (c) is known as the oxidising flame, and the inner luminous zone (b) the reducing flame. The two portions of the candle flame act in different manners on specific mineral substances, and the blowpipe operator may use either of them at will. The method of doing this is illustrated in the same figure. To obtain the reducing flame, the blowpipe jet is brought to the edge of the flame a little distance above the burner, or wick. The operator then produces a gentle blast, which deflects the latter (upper figure) without altogether passing into it, so that the flame is still charged with glowing carbon. A yellowish luminous flame is the result, the most active part of which lies at a short distance from the end.

On the other hand, the oxidising flame is utilised by passing the blowpipe jet a little farther into the flame (lower figure) and blowing more strongly. A pointed non-luminous flame is the result. This will be seen to possess an inner blue cone, before the point of which the hottest part is situated. Substances to be fused are placed in this part of the flame, whilst those to be oxidised are placed a little farther away, in order that they may be exposed to the air at the time they are being highly heated.

The “platinum wire” is an absolutely indispensable adjunct to a blowpipe outfit, and is employed as follows:—A short piece of the wire, an inch or so in length, being attached to a handle, as previously described, the free end of it is bent into a loop about the size of this O. This may be heated in the flame employed, or, better still, in the flame of a spirit lamp, and, when hot enough, it may be dipped into a small quantity of the powdered borax or microcosmic salt, some of which will be found to adhere to the wire. On further heating the borax it will swell out and form a number of irregular bubbles, which (heat still being applied) will subsequently settle down into a clear, colourless bead in the loop of the platinum wire. A satisfactory bead having now been made, a portion of the mineral substance to be analysed (in the shape of small grains) is taken up by dipping the heated borax bead therein.

The actual operation of determining the nature of the substance then commences. Using the blowpipe, and directing either the reducing flame (R.F.), or the oxidising flame (O.F.), on to the substance on the borax, according to circumstances presently to be detailed, the operator notes the change in colour (if any) of the flame yielded by the process. At this point a very annoying thing sometimes happens; for, in liquefying the borax bead, it is apt to fall off the wire, and another bead has then to be made. To avoid this, great care should be taken not to blow too vigorously at first. With the microcosmic salt especial care and dexterity must be exercised in this connection. If all goes well, the powdered mineral substance (if fusible in the borax) readily melts down, and becomes incorporated with the borax. On permitting the latter to cool, which it very rapidly does, the bead should now be carefully examined, and any change in tint noted. Most beautiful transparent colours, pregnant with meaning, are often seen to have formed with the borax as flux.

The operator may test his skill by making the following brilliant experiments. Take up a few small fragments of the mineral malachite (a carbonate of copper) by means of the clear, colourless, heated borax bead, and then introduce them to the oxidising flame. They slowly dissolve in the borax, and, whilst doing so, the tip of the blowpipe flame becomes emerald-green in colour. After applying this flame for a minute or two, the whole of the mineral will have become incorporated with the borax, and, when the bead is still hot, note that it is also of a rich green tint, but that, on cooling, it turns blue. If too much malachite has been taken up in the first instance, a very dark green tint is imparted, which still remains when the bead is cold, and it appears to be quite black. Its true colour, however, may be ascertained by flattening the bead out before it is quite cold. It is always well to begin by using a small quantity of the mineral substance at first, and adding to this as may be required.

Assuming that a fine rich green bead has been produced, and that it contains a relatively large amount of copper, the operator may now hold it in the reducing flame and re-melt the bead; if the operation has been conducted carefully, the bead will then show red, and be practically opaque when cold. The red bead may now be re-heated in the oxidising flame, when it will be found once more to return to a green colour. The student will find this easy operation excellent practice, as proving to him, in the absence of a demonstrator, that he is really able to recognise and use the oxidising and reducing flames at will. Many mineral substances yield a distinctive colour in this way—a useful factor in a qualitative analysis.

Before using the platinum wire, be careful to ascertain that it is quite clean; a borax bead made thereon should be perfectly white and transparent.

The “platinum foil” is employed as a support during fusions; pieces about an inch and a half long, by half an inch in width, are generally used. A small platinum spoon is sometimes adopted when fusing substances with acid, potassium, sulphate, or nitre.

Minerals may be tested to see whether, in the ordinary blowpipe flame, they are fusible or not. To do this, a fragment of the substance to be tested is held in the flame by means of the “platinum-pointed forceps.” If the mineral is found to be fusible, then its “degree of fusibility” may be ascertained according to the following table. The “degrees of fusibility” are six in number:—

1. Fusible in ordinary gasflame, even in large fragments. Example: Stibnite, or grey antimony.

2. Fusible in fine, thin pieces, in the ordinary gasflame, and in larger fragments in the blowpipe-flame. Example: Natrolite, a hydrous silicate of alumina and soda.

3. If very thin splinters be used, fusible without difficulty with the blowpipe-flame. Example: Almandite, or iron-alumina-garnet.

4. In thin splinters fusible to a globule. Example: Actinolite, a non-aluminous variety of hornblende.

5. Thin edges may be fused and rounded without great difficulty. Example: Orthoclase felspar—already described.

6. Fusible with great difficulty on the finest edges. Example: Bronzite, one of the augite group of minerals.

Now, it is highly probable that many of our readers will not understand, or be able to recognise the six minerals above enumerated; and we recommend those who may be sufficiently interested, to purchase them from a mineral dealer—such as Damon, of Weymouth, or Russell, or Gregory, or Henson, or Butler, in London. A set, comprising the six, should cost from two to three shillings. With these, as a standard for comparison, the operator readily grasps the method of assigning a fusible mineral to its proper degree in the scale.

Another object of examination in the forceps is to see what colour (if any) is imparted to the flame by the divers minerals experimented upon. It is a good rule not to permit the specimen, when being fused, to touch the forceps in the neighbourhood of the actual part fused. For a mineral containing antimony or arsenic would tend to form a fusible alloy with the platinum points, and so ruin the forceps.

The pieces of “charcoal” alluded to in our inventory, are used for placing the mineral substance upon in certain parts of the blowpipe operation, which may be briefly described. Essentially the charcoal forms a support to the substance during fusion; but the glowing carbon has also a kind of reducing effect. Taking a long prism of charcoal, such as that described, page 63 ante, the mineral to be dealt with should be placed near one end of a flat surface and the prism so held that the flame from the blowpipe, will sweep down its full length. The object of so doing is to give a chance to any volatile substance (derived by the operation from the mineral) to deposit on the comparatively cool surface, which deposit is often indicative of the chemical nature of the mineral. To carry this point home, the following experiments may be conducted by the student. Taking a piece of stibnite (sulphide of antimony), which, as we have just learnt, is a most fusible mineral, we place it on the charcoal in the manner indicated. Whilst melting, and the blowpipe flame be continued to be directed upon it after it has become fused, it will be noticed that a yellowish-white deposit is taking place on the length of charcoal; this is called a sublimate.

Mineral substances may also be assisted in fusing on the charcoal by using the reagents described in our list of chemicals, &c., included in a blowpipe set.

In regard to the use of the “glass tubes,” it may be remarked that they are used principally for the examination of minerals which yield a volatile substance on being heated therein, and to detect the presence of water and the like. It is important to make a distinction between the closed and the open tubes. When a mineral fragment is placed in a tube, closed at one end, whatever takes place will be in presence of very little air, or oxygen; on the other hand, when the tube is open at both ends, and is inclined during the experiment, a constant stream of oxygen passes through the tube, and the mineral is being dealt with in presence of that. The employment of this oxygen makes a great deal of difference in the results obtained, as a few elementary experiments will show. If we place a piece of sulphur in a tube, closed at one end, and heat it gently, we notice that a yellow coating takes place inside the tube; but if we now employ a tube open at both ends and heat it very slowly indeed, we notice that the sulphur goes off as an invisible gas, and if the experiment has been properly conducted, there should hardly be a trace of the sulphur left on the glass. A number of experiments of a similar nature might be quoted, but enough has been said for the present to show the utility of the tubes.

The “chemical reagents” alluded to have already been sufficiently described to render any further discussion on them unnecessary for our immediate purpose.

In regard to the “miscellaneous articles” mentioned, it may be remarked that the test papers are employed in the detection of certain acids and bases; whilst a strip of brazil-wood paper is for the detection of fluorine. The hammer and anvil are for breaking the substance to be tested into small fragments; the magnet for withdrawing particles of iron from the pulverised material; the three-cornered file for assisting in determining the relative hardness of minerals, &c., &c.

In examining substances before the blowpipe, it is highly desirable that the various operations should be carried out in some definite order. The following has been found convenient:—

a. In a glass tube closed at one end.
b. In an open tube.
c. On charcoal.
d. With borax and microcosmic salt.
e. As to flame colouration.
f. With other reagents.

The size of the fragment to be dealt with in an examination, depends on circumstances, but for ordinary purposes a piece of the size of a small rabbit-shot will be found sufficient.

It is convenient in this place to describe a few chemical reactions without the use of the blowpipe; that will render the effects on certain minerals, presently to be mentioned, clearer to the reader.

In the first place it may be ascertained whether the mineral is soluble in water, and if so, to what extent. Then as to whether it becomes soluble in certain acids, such as hydrochloric or nitric acid. The former acid is generally used, except for metallic sulphides, and those minerals containing heavy metals, such as lead, silver, &c.; the latter is employed for the exceptions named. Several minerals, even when in a powdered state, are hardly, if at all, affected by acids. The results to be noted during the test with acids, commonly fall into the following three groups.[7]

A. The mineral may dissolve quietly with or without colouring the solution; this holds good, for example, with hematite (a variety of iron), also of many of the sulphates and phosphates.

B. There may be a bubbling off or effervescence of a gas, which gas is usually carbon dioxide; but may be hydrogen sulphide. Chlorine may be liberated, or reddish fumes of nitrogen.

C. There may be separation of some insoluble substance as sulphur, silica, &c.

We will close this chapter by stating the behaviour under blowpipe examination of various minerals, given in preceding pages, as being common in clays and earths used in brickmaking:—

Quartz.—This is infusible, and remains undissolved, even in a microcosmic salt bead; but it fuses readily with soda, on charcoal. In the flame it splinters into fragments, which fly off with great rapidity. It is soluble in hydrofluoric acid. Flint, when pure, behaves in a similar manner.

Orthoclase Felspar.—Fusibility, 5; flame colouration brilliant yellow, when much sodium is present; not decomposed by hydrochloric acid. It may be distinguished from other common felspars by its high degree of fusibility.

Oligoclase Felspar.—Gives a sodium yellow flame; fusibility, 3.5; not decomposed by hydrochloric acid.

Biotite Mica.—With fluxes gives a strong iron reaction of yellowish red colour; decomposed in concentrated sulphuric acid, leaving a residue of siliceous matter.

Muscovite Mica.—When heated in a tube closed at one end, yields water which often gives fluorine reaction with brazil-wood test paper by colouring it straw-yellow; it is not decomposed by acids, and whitens and fuses only on thin edges.

Kaolin.—Is infusible; gives off water when heated in a closed tube; and with cobalt nitrate on charcoal, a fine alumina reaction is obtained.

Aluminium.—On charcoal, this becomes blue with cobalt nitrate, though if the surface is fused the reaction is not so clear. Prof. Cole advises that the soda-residue be dissolved in dilute hydrochloric acid, then evaporated to dryness, re-dissolved in that acid water, filter off any silica, and neutralise with ammonia; alumina is precipitated together with any iron present. The precipitate, if white, or nearly so, may be tested with cobalt nitrate, and the result is a fine blue colour.

Limonite Iron.—Fusibility about 5; yellow and reddish beads; water given off in closed tube; in reducing flame magnetic residue on charcoal; soluble in hydrochloric acid after a short time.

Iron Pyrites.—Fusibility about 2; yellow and red beads; in closed tube yellow precipitate due to sulphur; magnetic after reduction on charcoal; insoluble in hydrochloric acid.

Rock Salt.—Intense yellow sodium flame; fusibility about 1; microcosmic salt with copper oxide shows strong chlorine reaction—a fine blue flame surrounding the bead when re-introduced into the flame. It is soluble in water.

Selenite (Gypsum).—Fusibility about 2.5; brilliant flame; in closed tube it becomes white and opaque and much water is given off; with soda, on charcoal, sulphur reactions are obtained; soluble in hydrochloric acid.

Calcite (Carbonate of Lime).—Flame glows very strongly; infusible; effervesces freely in cold hydrochloric acid.

Dolomite.—Flame, with hydrochloric acid, like calcite; infusible; effervesces in hot hydrochloric acid.

Magnesite.—Infusible; with cobalt nitrate a fair magnesia reaction on charcoal, i.e., turns into a dull pink; effervesces in hot hydrochloric acid.

Manganese.—With borax in oxidising flame a red-violet bead is obtained, but with the reducing flame it is colourless.

The above are commonly met with in brick-earths; for other minerals and substances also found, the reader may be referred to special works dealing with blowpipe analysis.

CHAPTER VIII.
THE CHEMISTRY OF BRICK-EARTHS (Continued).

In this chapter we shall fulfil our promise (ante p. 58) to explain in an elementary manner the precise meaning of ordinary commercial chemical analyses of some typical earths used in brickmaking, etc. We may commence by explaining a few terms used by the chemist.

An atom is the smallest imaginable portion of matter, and all matter is said to consist of atoms. A molecule is the smallest conceivable combination of atoms, and every compound substance is ultimately built up of molecules. An element is a substance that has hitherto defied the efforts of the chemist to subdivide or split up. Over seventy of these elementary substances are at present known, and their number is being constantly added to. Again, by improvement in analytical methods, a so-called element may be subdivided, and thus removed from the list. The elements are classified into metals and non-metals; and it is convenient to give each of them a symbol to save trouble in writing, and to render clearer to the reader the chemical nature of a compound body. Thus, the symbol for the element aluminium is Al; for silicon Si; for carbon C; for calcium Ca; for oxygen O; for iron Fe; for hydrogen H; for chlorine Cl; and so on.

We are taught by chemistry that elements are capable of combining only in definite proportions, and that each substance possesses a definite proportion peculiar to itself. That proportion is called the atomic weight of the element; or, it is the relative weight of the atom of each substance compared with that of the lightest substance known, hydrogen.

Thus, the atomic weight of hydrogen being taken as 1, it is found that an atom of chlorine is 35.5 times as heavy as that, so that the atomic weight of chlorine is said to be 35.5. Now, in spite of the enormous difference between the weight of the two elements just mentioned, they combine in the same proportions by volume; and the union is known as hydrochloric acid, or HCl.

But in certain cases elements do not combine in equal proportions; for instance, an atom of oxygen will not combine with less than two of hydrogen. Further, with this we find that the three volumes are condensed into the space of two volumes—a very common phenomenon in the chemical combination of gases. The union of hydrogen and oxygen alluded to forms water, the chemical symbol of which is, consequently, H₂O.

Chemical affinity, or chemical attraction, is the force which is exerted between molecules not of the same kind. Thus, in water, which, as we have seen, is composed of hydrogen and oxygen, it is affinity which unites these elements, but it is cohesion which binds together two molecules of water. In compound bodies, cohesion and affinity operate simultaneously; whilst in simple bodies, or elements, cohesion alone has to be considered. To affinity are due all the phenomena of combustion and of chemical combination and decomposition.

Certain gases, such as chlorine and nitrogen, and such substances as sulphur, carbon, and silicon, with many others, form acids in conjunction with hydrogen, or hydrogen and oxygen. These combine with greater or less facility with other elements which do not form acids, and are termed bases. A combination of an acid and a base is known as a salt. Salts the names of which end in -ide, such as chloride, sulphide, etc., are combinations of a metal with a non-metal. Monoxide means an oxide containing one atom of oxygen; dioxide one containing two atoms; protoxide means the first oxide, because it is the first or lowest of the oxides of the given metal in amount of oxygen present; the highest oxide is often known as peroxide. The terminations -ous and -ic are frequently used for the lower and higher oxides respectively. Examples:—

FeO, iron protoxide, or ferrous oxide.
Fe_{2}O_{3}, iron sesquioxide, or ferric oxide.
FeS_{2}, iron disulphide.
Sb₂S₃, antimony trisulphide.

The following symbols may be indicated as referring to compounds especially met with in brick-earths:—

CaO, lime, instead of calcium oxide.
Al_{2}O_{3}, alumina, instead of aluminium trioxide.
SiO_{2}, silica, instead of silicon dioxide.
Na_{2}O, soda, instead of sodium oxide.
K_{2}O, potash, instead of potassium oxide.
MgO, magnesia, instead of magnesium oxide.

In analysing a body, the first step consists in determining the nature of the elementary substances contained therein. That may be accomplished in the dry way by means of the blowpipe and accessories, as explained in the last chapter. Such an examination, as previously remarked, is known as a qualitative analysis. Or, it may be accomplished in the wet way by ordinary chemical examination. The next step is to determine the amount of the constituents present, and that is known as a quantitative analysis. In making a qualitative analysis, the chemist is assisted by the knowledge that certain basic substances and certain acids produce peculiar phenomena in the presence of known substances or preparations termed reagents.

There is a great difference between a chemical compound and a simple mixture of elements; and it is not always easy (e.g., some alloys) to say whether a substance is in the one state or the other. This distinction is well exemplified by the air we breathe. The chemist finds by analysis that the air is nearly constant in composition, containing essentially in 100 parts 76.8 by weight of nitrogen (including about 1 per cent. of the recently-discovered element, argon), and 23.2 of oxygen. Small proportions of water vapour, carbon dioxide, etc., may be ignored for our present purposes. In view of this comparatively uniform composition, the question at first arises as to whether the air is, or is not, a chemical compound? The answer is in the negative, for, amongst other things, it can be shown that the ratio of 76.8 to 23.2 is not that of the atomic weights of the two elements present, viz., 14 : 16, nor of any simple multiples of these.

We will now quote a few analyses of well-known earths, and explain each in turn:

Chemical Composition of China-clays.[8]

The kaolin alluded to in the first column is a remarkably pure material, perfectly white, and contains an enormous quantity of water. It refers to one of the finest washed china-clays in the market, and is extensively used in porcelain manufacture. It is quoted here principally to give an idea of what a really pure clay is like chemically. We notice that, in spite of its relative purity, it contains .27 per cent. of iron oxide. This could have been well done without, from the manufacturer’s standpoint, but is of course a very minute proportion. Small as it is, it must exert a slight amount of colouring influence. The lime and magnesia are present in slightly larger proportions, and a little more of either would be advantageous rather than otherwise, as assisting to flux the material. This is an earth with which practically anything may be done by judicious blending and careful preparation.

With reference to the second column, the figures do not refer to any particular clay, but they have been compiled to show the average composition of kaolins as used in the market. It will be observed that the silica and alumina are present in approximately equal proportions, which is a characteristic of fairly good china-clays. The iron oxide remains as before, but there is a larger proportion of lime and magnesia—as much as can be permitted except in a second-rate clay.

The evidence of the third column shows that the sand in the china-clay is to a large extent quartzose, and this is at the expense of the alumina. Such a material would be suitable for making a species of white fire-brick, and it might do for the commoner kinds of china-ware. The earth is really of the nature of a loam—a sandy clay. There is too much iron in it for the production of perfectly white goods. The proportion of lime might be increased to advantage.

Chemical Composition of Fire-clays from Newcastle-on-Tyne.[9]

The reader will see at a glance that the range of variation permissible in fire-clays is very wide. These earths are all found close together, and are utilised for similar purposes, though often blended to produce desired results. It will be noticed that one of them (No. 6) contains as much as 83.29 of silica, whilst another has no more than 47.55 per cent. The range with reference to alumina is very wide also, from 8.10 percent. (No. 6) to 31.35. The refractory character of any sample of fire-clay is determined by the proportions in which the silica and alumina are contained, and by the absence of lime, iron, and other easily fluxible substances. The proportion of iron discovered in sample No. 2 is certainly much in excess of the requirements of the material, as a fire-clay, and this no doubt is tempered by admixture, unless utilised for inferior goods. The iron oxide in the other samples is about sufficient for general purposes. The amount of lime present in all the samples constitutes a good feature; much lime cannot on any account be allowed in earths for fire-clay goods. With so much iron present, and the fair proportions of magnesia (except in sample No. 4) these clays may be regarded as typical, with the exception of No. 6. They have been utilised for many years in the manufacture of fire-bricks and the like.

Chemical Composition of Fire-clays, from Welsh localities.

The first thing that will strike the reader on looking at these results on Welsh materials, is their uniform composition as compared with the clays from Newcastle. Yet there is as much as 10.40 per cent. of iron in sample No. 1, which cannot be a first-rate clay. Its proportion of silica to alumina is, however, excellent, and, as in sample No. 3, the amount of soda and magnesia is not excessive. The soda in sample No. 2 (which acts somewhat like lime in the kiln) taken together with the magnesia and iron in the same material, is too much for a first-class clay, and would have to be suitably modified before good results could be obtained. On the whole, it is possible that sample No. 3 would yield the best results from the chemical standpoint.

We should not forget that remarkable substance of which the well-known Dinas bricks are made. The proportion of silica present ranges from about 96 to 99 per cent., the remainder consisting principally of alumina, though traces of iron, lime, and magnesia frequently occur. There is not, of course, sufficient natural flux for this “clay,” so a small proportion (2.5 to 3 per cent.) of lime is added, which produces the desired effect. In other words, if we can obtain a pure siliceous sand, with hardly any lime, iron or magnesia in it, we have the material of which the better kinds of fire-bricks are made. Such sandy earths are not uncommon in the South of England, but strange to relate, they are not used for the purpose indicated.

The earths from which the superior Stourbridge bricks are made, are approximately of the following chemical composition:—Silica, 64.10; alumina, 23.15; iron oxide, 1.85; magnesia, .95; water and loss, 10.00 per cent. It will be observed that the proportion of iron and magnesia here is very small, whilst lime is altogether absent. It is a most excellent earth for the purposes for which it is used, and the chemical results may be taken as a standard for that class of material. Another Stourbridge earth yields as much as 4.14 per cent. of iron, however, whilst its proportion of silica is lower, 51.80, and alumina higher, 30.40, which serves to remind us of the variability of even good earths used in the manufacture of fire-clay goods.

Let us now turn to the consideration of pottery clays, of which the following results may be taken as typical:—

Chemical Composition of Pottery Clays.

Some of the chief qualifications, from a chemical point of view, of earths suitable for making pottery, is the proportion and potentiality of the colouring matters present. Where the pottery is to be glazed, that is not so important; but with ordinary unglazed ware, colour and uniformity are two highly essential desiderata. We know that the temperature employed will modify the tint, but under similar conditions the clays alluded to in the above table will give, approximately, the following results. Sample No. 1 is typical of an excellent blue pottery clay, which burns white. It contains more alumina than is commonly met with in such materials, in which respect it differs markedly also from the fire-clays just described. The proportion of oxide of iron is very small, not sufficient to perceptibly colour the finished product, though, no doubt, on careful examination it would be seen not to be perfectly white. The latitude of the term “white” is pretty considerable with clayworkers, as the reader is probably aware.

The pottery clay (also used for bricks) referred to in the middle column, is brown in colour; it is an ordinary kind, used primarily for black and common red ware. The proportion of iron is high, and considerable quantities of both lime and magnesia exist. As might naturally be expected of such material, it will not bear exposure to great heat, though that might be regarded as a qualification in some brick and pottery yards.

The proportion of silica is high in sample No. 3, which appertains to a common yellow clay, with, possibly, some siliceous sand in it. The amount of alumina is correspondingly low, but the iron oxide is not excessive—for a common pottery clay. It is used for the manufacture of coarse ware, and burns yellow.

The chemical composition of earths used for terra-cotta and bricks of that substance is so variable, that without going into each case specifically it would be impossible to convey an adequate idea. It may be stated generally that it is not one whit less important to consider the composition of the raw earths for ordinary brickmaking, than in respect of that for high-class bricks and pottery.

An excellent earth, from the neighbourhood of Ruabon, is of the following composition:—

Chemical Composition of Ruabon Clay.

The proportion of silica in this is higher than in many clays used for brick- or terra-cotta making, but the alkalis, potash and soda, are in strong force, so that any refractoriness on the part of the silica is soon subdued in the kiln. The iron, also, is in abundance. The principal colouring ingredient is the sesquioxide, and we can quite understand the manufacturer when he informs us that, in spite of the rich tint of the goods produced, nothing is artificially mixed with this clay to produce such a result. We may call attention to the method of expressing the chemical analysis in this case, which might be copied to advantage. In the first place, the combined and the uncombined iron are separately shown, or rather the degree of combination is indicated; and secondly, the proportion of water chemically combined is differentiated from that which has simply soaked into the clay, though expelled, following a well-known practice of chemists, prior to commencing the analysis proper. It is of very little use giving the amount of water, unless the proportions are divided in this manner. In the result given above we learn that there is very little chance of the clay shrinking, as it only contains moisture to the extent of 1.54 per cent.; but if that had been added to the water combined, we should have had a result of 5.08 per cent., which is not nearly so clear in its meaning. We may add that the Ruabon earth referred to is utilised also in the manufacture of tesselated and encaustic tiles.

In regard to the composition of earths employed in the manufacture of the commoner kinds of bricks, we may give the following examples:—

Chemical Composition of Common Brick-earths.

Reviewing these results, it will be noted that the brown colouring imparted to the brick in the first-mentioned example is due, to a large extent, to the presence of manganese, a rather uncommon feature in brick-earths, except where these have resulted from the denudation of iron-producing rocks rich in manganese. It will be noticed also that the proportion of water is not high for a common earth, and it must be a fairly easy material to deal with. There seem to be some possibilities in it that might, in competent hands, lead to higher things. The amount of lime and magnesia is, however, a rather serious one for a first-class clay.

In regard to the “red-brick” clay, an essential feature is the comparative absence of lime, and it would, no doubt, make “rubbers” of an ordinary kind. Unfortunately, in the results given, the iron is not separated from the alumina, but clearly the latter is very small in amount, and the results refer to a sandy material. The proportion of water is disastrous for the employment of this earth by unskilful hands. In drying, the greatest care would have to be exercised to prevent undue shrinking, and, in any case, the earth would have to be very thoroughly incorporated to make a really serviceable brick. It is with earths of this character that the majority of brickmakers in embryo come to grief; they know not how to handle them successfully, and twisting, warping, cracking, and “bursting” follow as a natural consequence. It is a common and treacherous material, that could only be made to succeed by perseverance and wide experience.

The “common brick-earth,” as will be seen, contains an abnormal quantity of lime, and doubtless refers to a marl, though not much alumina is shown. Malm bricks could be made from it, and the product would have to be burned at a low temperature. For bricks useful to the “jerry-builder” this earth could be strongly recommended. It was, no doubt, mainly derived from limestone rocks; and, judging from the high proportion of magnesia, probably from within a watershed composed to some extent of magnesian limestone.

The “sandy-clay” or loam is of a very common type, and produces light-red bricks. There is much in common between this and the “red-brick clay” previously referred to.

The practice resorted to in various parts of the world of making bricks from slate débris, although not hitherto adopted to any large extent in this country, merits some description in this place. Slates may be regarded as a highly compressed clay, the original structure of which has been materially modified by the great pressure exerted during their manufacture in Nature’s laboratory. To all intents and purposes they are silicates of alumina, plus iron, lime, magnesia, and so on, and have, practically, the same range of variation as have ordinary clays. But during their manufacture, and subsequently, certain adventitious mineral matter has been frequently introduced, as may be gathered from the following results:—

Chemical Composition of Slates.

Analysis No. 1 refers to a blue Welsh roofing slate of Cambrian age. It is quite certain that the large proportion of alkalis present would render this material unsuitable for brickmaking, except for the commonest kinds of bricks. The iron, again, is very large in quantity, whilst the amount of alumina is low. We could not recommend this slate for good bricks under any consideration.

Analysis No. 2 is of a dark-blue slate from Llangynog, in North Wales. The amount of iron present is high, but from the low content of alkalis this material, under proper treatment, should make fairly good bricks. The ferruginous constituent is too powerful, however, for fire-bricks to be made of this slate.

Analysis No. 3, of a purple slate from Nantlle, shows a remarkable diminution in silica and a corresponding increase in iron. Lime and magnesia being present to such an enormous extent, taken in conjunction with the iron, would render this slate absolutely useless for brickmaking. There is not a redeeming feature about it.

Analysis No. 4, which refers to a green Westmorland slate, has a low percentage of alumina and very large quantities of iron, lime, and magnesia. Only bricks of an exceedingly inferior quality could result from such material.

Summing up the general characteristics of these slates from the chemical aspect, one would say that none of them are very suitable for high-class bricks. No. 2 is the best. Several minor differences will be observed between the results quoted and those referring to ordinary brick-earths—in particular, the distribution of the alkalis. A general impression is abroad that any purple slate will do for brickmaking, and manufacturers do not yet seem to have realised that the chemical nature of slates is as variable as of brick-earths. That may account for the difficulties experienced in many cases in turning out a satisfactory material. The microscope is of much use in this connexion, however, and the practical effects of chemical analyses are not always as bad as they seem at first sight.

The remainder of this chapter will be devoted to the consideration of rarer kinds of brick-earth and other raw earths used principally in the manufacture of bricks for special purposes, or as pointing to certain anomalies. As an example of what some manufacturers can do, we may quote the chemical composition of a peculiar brick-earth employed in Zurich, in Switzerland:—